// Copyright (c) 2019, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:_fe_analyzer_shared/src/flow_analysis/flow_analysis.dart';
import 'package:test/test.dart';

import 'flow_analysis_mini_ast.dart';

main() {
  group('API', () {
    test('asExpression_end promotes variables', () {
      var h = Harness();
      var x = Var('x', 'int?');
      late SsaNode<Var, Type> ssaBeforePromotion;
      h.run([
        declare(x, initialized: true),
        getSsaNodes((nodes) => ssaBeforePromotion = nodes[x]!),
        x.read.as_('int').stmt,
        checkPromoted(x, 'int'),
        getSsaNodes((nodes) => expect(nodes[x], same(ssaBeforePromotion))),
      ]);
    });

    test('asExpression_end handles other expressions', () {
      var h = Harness();
      h.run([
        expr('Object').as_('int').stmt,
      ]);
    });

    test('assert_afterCondition promotes', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        assert_(x.read.eq(nullLiteral),
            checkPromoted(x, 'int').thenExpr(expr('String'))),
      ]);
    });

    test('assert_end joins previous and ifTrue states', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('x', 'int?');
      var z = Var('x', 'int?');
      h.run([
        x.read.as_('int').stmt,
        z.read.as_('int').stmt,
        assert_(block([
          x.write(expr('int?')).stmt,
          z.write(expr('int?')).stmt,
        ]).thenExpr(x.read.notEq(nullLiteral).and(y.read.notEq(nullLiteral)))),
        // x should be promoted because it was promoted before the assert, and
        // it is re-promoted within the assert (if it passes)
        checkPromoted(x, 'int'),
        // y should not be promoted because it was not promoted before the
        // assert.
        checkNotPromoted(y),
        // z should not be promoted because it is demoted in the assert
        // condition.
        checkNotPromoted(z),
      ]);
    });

    test('conditional_thenBegin promotes true branch', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read
            .notEq(nullLiteral)
            .conditional(checkPromoted(x, 'int').thenExpr(expr('int')),
                checkNotPromoted(x).thenExpr(expr('int')))
            .stmt,
        checkNotPromoted(x),
      ]);
    });

    test('conditional_elseBegin promotes false branch', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read
            .eq(nullLiteral)
            .conditional(checkNotPromoted(x).thenExpr(expr('Null')),
                checkPromoted(x, 'int').thenExpr(expr('Null')))
            .stmt,
        checkNotPromoted(x),
      ]);
    });

    test('conditional_end keeps promotions common to true and false branches',
        () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      var z = Var('z', 'int?');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        declare(z, initialized: true),
        expr('bool')
            .conditional(
                block([
                  x.read.as_('int').stmt,
                  y.read.as_('int').stmt,
                ]).thenExpr(expr('Null')),
                block([
                  x.read.as_('int').stmt,
                  z.read.as_('int').stmt,
                ]).thenExpr(expr('Null')))
            .stmt,
        checkPromoted(x, 'int'),
        checkNotPromoted(y),
        checkNotPromoted(z),
      ]);
    });

    test('conditional joins true states', () {
      // if (... ? (x != null && y != null) : (x != null && z != null)) {
      //   promotes x, but not y or z
      // }
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      var z = Var('z', 'int?');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        declare(z, initialized: true),
        if_(
            expr('bool').conditional(
                x.read.notEq(nullLiteral).and(y.read.notEq(nullLiteral)),
                x.read.notEq(nullLiteral).and(z.read.notEq(nullLiteral))),
            [
              checkPromoted(x, 'int'),
              checkNotPromoted(y),
              checkNotPromoted(z),
            ]),
      ]);
    });

    test('conditional joins false states', () {
      // if (... ? (x == null || y == null) : (x == null || z == null)) {
      // } else {
      //   promotes x, but not y or z
      // }
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      var z = Var('z', 'int?');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        declare(z, initialized: true),
        if_(
            expr('bool').conditional(
                x.read.eq(nullLiteral).or(y.read.eq(nullLiteral)),
                x.read.eq(nullLiteral).or(z.read.eq(nullLiteral))),
            [],
            [
              checkPromoted(x, 'int'),
              checkNotPromoted(y),
              checkNotPromoted(z),
            ]),
      ]);
    });

    test('declare() sets Ssa', () {
      var h = Harness();
      var x = Var('x', 'Object');
      h.run([
        declare(x, initialized: false),
        getSsaNodes((nodes) {
          expect(nodes[x], isNotNull);
        }),
      ]);
    });

    test('equalityOp(x != null) promotes true branch', () {
      var h = Harness();
      var x = Var('x', 'int?');
      late SsaNode<Var, Type> ssaBeforePromotion;
      h.run([
        declare(x, initialized: true),
        getSsaNodes((nodes) => ssaBeforePromotion = nodes[x]!),
        if_(x.read.notEq(nullLiteral), [
          checkReachable(true),
          checkPromoted(x, 'int'),
          getSsaNodes((nodes) => expect(nodes[x], same(ssaBeforePromotion))),
        ], [
          checkReachable(true),
          checkNotPromoted(x),
          getSsaNodes((nodes) => expect(nodes[x], same(ssaBeforePromotion))),
        ]),
      ]);
    });

    test('equalityOp(x != null) when x is non-nullable', () {
      var h = Harness();
      var x = Var('x', 'int');
      h.run([
        declare(x, initialized: true),
        if_(x.read.notEq(nullLiteral), [
          checkReachable(true),
          checkNotPromoted(x),
        ], [
          checkReachable(true),
          checkNotPromoted(x),
        ])
      ]);
    });

    test('equalityOp(<expr> == <expr>) has no special effect', () {
      var h = Harness();
      h.run([
        if_(expr('int?').eq(expr('int?')), [
          checkReachable(true),
        ], [
          checkReachable(true),
        ]),
      ]);
    });

    test('equalityOp(<expr> != <expr>) has no special effect', () {
      var h = Harness();
      h.run([
        if_(expr('int?').notEq(expr('int?')), [
          checkReachable(true),
        ], [
          checkReachable(true),
        ]),
      ]);
    });

    test('equalityOp(x != <null expr>) does not promote', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        if_(x.read.notEq(expr('Null')), [
          checkNotPromoted(x),
        ], [
          checkNotPromoted(x),
        ]),
      ]);
    });

    test('equalityOp(x == null) promotes false branch', () {
      var h = Harness();
      var x = Var('x', 'int?');
      late SsaNode<Var, Type> ssaBeforePromotion;
      h.run([
        declare(x, initialized: true),
        getSsaNodes((nodes) => ssaBeforePromotion = nodes[x]!),
        if_(x.read.eq(nullLiteral), [
          checkReachable(true),
          checkNotPromoted(x),
          getSsaNodes((nodes) => expect(nodes[x], same(ssaBeforePromotion))),
        ], [
          checkReachable(true),
          checkPromoted(x, 'int'),
          getSsaNodes((nodes) => expect(nodes[x], same(ssaBeforePromotion))),
        ]),
      ]);
    });

    test('equalityOp(x == null) when x is non-nullable', () {
      var h = Harness();
      var x = Var('x', 'int');
      h.run([
        declare(x, initialized: true),
        if_(x.read.eq(nullLiteral), [
          checkReachable(true),
          checkNotPromoted(x),
        ], [
          checkReachable(true),
          checkNotPromoted(x),
        ])
      ]);
    });

    test('equalityOp(null != x) promotes true branch', () {
      var h = Harness();
      var x = Var('x', 'int?');
      late SsaNode<Var, Type> ssaBeforePromotion;
      h.run([
        declare(x, initialized: true),
        getSsaNodes((nodes) => ssaBeforePromotion = nodes[x]!),
        if_(nullLiteral.notEq(x.read), [
          checkPromoted(x, 'int'),
          getSsaNodes((nodes) => expect(nodes[x], same(ssaBeforePromotion))),
        ], [
          checkNotPromoted(x),
          getSsaNodes((nodes) => expect(nodes[x], same(ssaBeforePromotion))),
        ]),
      ]);
    });

    test('equalityOp(<null expr> != x) does not promote', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        if_(expr('Null').notEq(x.read), [
          checkNotPromoted(x),
        ], [
          checkNotPromoted(x),
        ]),
      ]);
    });

    test('equalityOp(null == x) promotes false branch', () {
      var h = Harness();
      var x = Var('x', 'int?');
      late SsaNode<Var, Type> ssaBeforePromotion;
      h.run([
        declare(x, initialized: true),
        getSsaNodes((nodes) => ssaBeforePromotion = nodes[x]!),
        if_(nullLiteral.eq(x.read), [
          checkNotPromoted(x),
          getSsaNodes((nodes) => expect(nodes[x], same(ssaBeforePromotion))),
        ], [
          checkPromoted(x, 'int'),
          getSsaNodes((nodes) => expect(nodes[x], same(ssaBeforePromotion))),
        ]),
      ]);
    });

    test('equalityOp(null == null) equivalent to true', () {
      var h = Harness();
      h.run([
        if_(expr('Null').eq(expr('Null')), [
          checkReachable(true),
        ], [
          checkReachable(false),
        ]),
      ]);
    });

    test('equalityOp(null != null) equivalent to false', () {
      var h = Harness();
      h.run([
        if_(expr('Null').notEq(expr('Null')), [
          checkReachable(false),
        ], [
          checkReachable(true),
        ]),
      ]);
    });

    test('equalityOp(null == non-null) is not equivalent to false', () {
      var h = Harness();
      h.run([
        if_(expr('Null').eq(expr('int')), [
          checkReachable(true),
        ], [
          checkReachable(true),
        ]),
      ]);
    });

    test('equalityOp(null != non-null) is not equivalent to true', () {
      var h = Harness();
      h.run([
        if_(expr('Null').notEq(expr('int')), [
          checkReachable(true),
        ], [
          checkReachable(true),
        ]),
      ]);
    });

    test('equalityOp(non-null == null) is not equivalent to false', () {
      var h = Harness();
      h.run([
        if_(expr('int').eq(expr('Null')), [
          checkReachable(true),
        ], [
          checkReachable(true),
        ]),
      ]);
    });

    test('equalityOp(non-null != null) is not equivalent to true', () {
      var h = Harness();
      h.run([
        if_(expr('int').notEq(expr('Null')), [
          checkReachable(true),
        ], [
          checkReachable(true),
        ]),
      ]);
    });

    test('conditionEqNull() does not promote write-captured vars', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        if_(x.read.notEq(nullLiteral), [
          checkPromoted(x, 'int'),
        ]),
        localFunction([
          x.write(expr('int?')).stmt,
        ]),
        if_(x.read.notEq(nullLiteral), [
          checkNotPromoted(x),
        ]),
      ]);
    });

    test('declare(initialized: false) assigns new SSA ids', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      h.run([
        declare(x, initialized: false),
        declare(y, initialized: false),
        getSsaNodes((nodes) => expect(nodes[y], isNot(nodes[x]))),
      ]);
    });

    test('declare(initialized: true) assigns new SSA ids', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        getSsaNodes((nodes) => expect(nodes[y], isNot(nodes[x]))),
      ]);
    });

    test('doStatement_bodyBegin() un-promotes', () {
      var h = Harness();
      var x = Var('x', 'int?');
      late SsaNode<Var, Type> ssaBeforeLoop;
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        checkPromoted(x, 'int'),
        getSsaNodes((nodes) => ssaBeforeLoop = nodes[x]!),
        branchTarget((t) => do_([
              getSsaNodes((nodes) => expect(nodes[x], isNot(ssaBeforeLoop))),
              checkNotPromoted(x),
              x.write(expr('Null')).stmt,
            ], expr('bool'))),
      ]);
    });

    test('doStatement_bodyBegin() handles write captures in the loop', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        do_([
          x.read.as_('int').stmt,
          // The promotion should have no effect, because the second time
          // through the loop, x has been write-captured.
          checkNotPromoted(x),
          localFunction([
            x.write(expr('int?')).stmt,
          ]),
        ], expr('bool')),
      ]);
    });

    test('doStatement_conditionBegin() joins continue state', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        branchTarget((t) => do_(
                [
                  if_(x.read.notEq(nullLiteral), [
                    continue_(t),
                  ]),
                  return_(),
                  checkReachable(false),
                  checkNotPromoted(x),
                ],
                block([
                  checkReachable(true),
                  checkPromoted(x, 'int'),
                ]).thenExpr(expr('bool')))),
      ]);
    });

    test('doStatement_end() promotes', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        branchTarget((t) =>
            do_([], checkNotPromoted(x).thenExpr(x.read.eq(nullLiteral)))),
        checkPromoted(x, 'int'),
      ]);
    });

    test('equalityOp_end on property get preserves target variable', () {
      // This is a regression test for a mistake made during the implementation
      // of "why not promoted" functionality: when storing information about an
      // attempt to promote a field (e.g. `x.y != null`) we need to make sure we
      // don't wipe out information about the target variable (`x`).
      var h = Harness();
      var x = Var('x', 'C');
      h.run([
        declare(x, initialized: true),
        checkAssigned(x, true),
        if_(x.read.propertyGet('y').notEq(nullLiteral), [
          checkAssigned(x, true),
        ], [
          checkAssigned(x, true),
        ]),
      ]);
    });

    test('equalityOp_end does not set reachability for `this`', () {
      var h = Harness()
        ..addSubtype('Null', 'C', false)
        ..addFactor('C', 'Null', 'C');
      h.addSubtype('C', 'Object', true);
      h.run([
        if_(this_('C').is_('Null'), [
          if_(this_('C').eq(nullLiteral), [
            checkReachable(true),
          ], [
            checkReachable(true),
          ]),
        ]),
      ]);
    });

    group('equalityOp_end does not set reachability for property gets', () {
      test('on a variable', () {
        var h = Harness();
        var x = Var('x', 'C');
        h.run([
          declare(x, initialized: true),
          if_(x.read.propertyGet('f').is_('Null'), [
            if_(x.read.propertyGet('f').eq(nullLiteral), [
              checkReachable(true),
            ], [
              checkReachable(true),
            ]),
          ]),
        ]);
      });

      test('on an arbitrary expression', () {
        var h = Harness();
        h.run([
          if_(expr('C').propertyGet('f').is_('Null'), [
            if_(expr('C').propertyGet('f').eq(nullLiteral), [
              checkReachable(true),
            ], [
              checkReachable(true),
            ]),
          ]),
        ]);
      });

      test('on explicit this', () {
        var h = Harness();
        h.run([
          if_(this_('C').propertyGet('f').is_('Null'), [
            if_(this_('C').propertyGet('f').eq(nullLiteral), [
              checkReachable(true),
            ], [
              checkReachable(true),
            ]),
          ]),
        ]);
      });

      test('on implicit this/super', () {
        var h = Harness();
        h.run([
          if_(thisOrSuperPropertyGet('f').is_('Null'), [
            if_(thisOrSuperPropertyGet('f').eq(nullLiteral), [
              checkReachable(true),
            ], [
              checkReachable(true),
            ]),
          ]),
        ]);
      });
    });

    test('finish checks proper nesting', () {
      var h = Harness();
      var e = expr('Null');
      var s = if_(e, []);
      var flow = FlowAnalysis<Node, Statement, Expression, Var, Type>(
          h, AssignedVariables<Node, Var>());
      flow.ifStatement_conditionBegin();
      flow.ifStatement_thenBegin(e, s);
      expect(() => flow.finish(), _asserts);
    });

    test('for_conditionBegin() un-promotes', () {
      var h = Harness();
      var x = Var('x', 'int?');
      late SsaNode<Var, Type> ssaBeforeLoop;
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        checkPromoted(x, 'int'),
        getSsaNodes((nodes) => ssaBeforeLoop = nodes[x]!),
        for_(
            null,
            block([
              checkNotPromoted(x),
              getSsaNodes((nodes) => expect(nodes[x], isNot(ssaBeforeLoop))),
            ]).thenExpr(expr('bool')),
            null,
            [
              x.write(expr('int?')).stmt,
            ]),
      ]);
    });

    test('for_conditionBegin() handles write captures in the loop', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        checkPromoted(x, 'int'),
        for_(
            null,
            block([
              x.read.as_('int').stmt,
              checkNotPromoted(x),
              localFunction([
                x.write(expr('int?')).stmt,
              ]),
            ]).thenExpr(expr('bool')),
            null,
            []),
      ]);
    });

    test('for_conditionBegin() handles not-yet-seen variables', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      h.run([
        declare(y, initialized: true),
        y.read.as_('int').stmt,
        for_(null, declare(x, initialized: true).thenExpr(expr('bool')), null, [
          x.write(expr('Null')).stmt,
        ]),
      ]);
    });

    test('for_bodyBegin() handles empty condition', () {
      var h = Harness();
      h.run([
        for_(null, null, checkReachable(true).thenExpr(expr('Null')), []),
        checkReachable(false),
      ]);
    });

    test('for_bodyBegin() promotes', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        for_(declare(x, initialized: true), x.read.notEq(nullLiteral), null, [
          checkPromoted(x, 'int'),
        ]),
      ]);
    });

    test('for_bodyBegin() can be used with a null statement', () {
      // This is needed for collection elements that are for-loops.
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        for_(declare(x, initialized: true), x.read.notEq(nullLiteral), null, [],
            forCollection: true),
      ]);
    });

    test('for_updaterBegin() joins current and continue states', () {
      // To test that the states are properly joined, we have three variables:
      // x, y, and z.  We promote x and y in the continue path, and x and z in
      // the current path.  Inside the updater, only x should be promoted.
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      var z = Var('z', 'int?');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        declare(z, initialized: true),
        branchTarget((t) => for_(
                null,
                expr('bool'),
                block([
                  checkPromoted(x, 'int'),
                  checkNotPromoted(y),
                  checkNotPromoted(z),
                ]).thenExpr(expr('Null')),
                [
                  if_(expr('bool'), [
                    x.read.as_('int').stmt,
                    y.read.as_('int').stmt,
                    continue_(t),
                  ]),
                  x.read.as_('int').stmt,
                  z.read.as_('int').stmt,
                ])),
      ]);
    });

    test('for_end() joins break and condition-false states', () {
      // To test that the states are properly joined, we have three variables:
      // x, y, and z.  We promote x and y in the break path, and x and z in the
      // condition-false path.  After the loop, only x should be promoted.
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      var z = Var('z', 'int?');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        declare(z, initialized: true),
        branchTarget((t) => for_(
                null, x.read.eq(nullLiteral).or(z.read.eq(nullLiteral)), null, [
              if_(expr('bool'), [
                x.read.as_('int').stmt,
                y.read.as_('int').stmt,
                break_(t),
              ]),
            ])),
        checkPromoted(x, 'int'),
        checkNotPromoted(y),
        checkNotPromoted(z),
      ]);
    });

    test('for_end() with break updates Ssa of modified vars', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('x', 'int?');
      late SsaNode<Var, Type> xSsaInsideLoop;
      late SsaNode<Var, Type> ySsaInsideLoop;
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        branchTarget((t) => for_(null, expr('bool'), null, [
              x.write(expr('int?')).stmt,
              if_(expr('bool'), [break_(t)]),
              getSsaNodes((nodes) {
                xSsaInsideLoop = nodes[x]!;
                ySsaInsideLoop = nodes[y]!;
              }),
            ])),
        getSsaNodes((nodes) {
          // x's Ssa should have been changed because of the join at the end of
          // of the loop.  y's should not, since it retains the value it had
          // prior to the loop.
          expect(nodes[x], isNot(xSsaInsideLoop));
          expect(nodes[y], same(ySsaInsideLoop));
        }),
      ]);
    });

    test(
        'for_end() with break updates Ssa of modified vars when types were '
        'tested', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('x', 'int?');
      late SsaNode<Var, Type> xSsaInsideLoop;
      late SsaNode<Var, Type> ySsaInsideLoop;
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        branchTarget((t) => for_(null, expr('bool'), null, [
              x.write(expr('int?')).stmt,
              if_(expr('bool'), [break_(t)]),
              if_(x.read.is_('int'), []),
              getSsaNodes((nodes) {
                xSsaInsideLoop = nodes[x]!;
                ySsaInsideLoop = nodes[y]!;
              }),
            ])),
        getSsaNodes((nodes) {
          // x's Ssa should have been changed because of the join at the end of
          // the loop.  y's should not, since it retains the value it had prior
          // to the loop.
          expect(nodes[x], isNot(xSsaInsideLoop));
          expect(nodes[y], same(ySsaInsideLoop));
        }),
      ]);
    });

    test('forEach_bodyBegin() un-promotes', () {
      var h = Harness();
      var x = Var('x', 'int?');
      late SsaNode<Var, Type> ssaBeforeLoop;
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        checkPromoted(x, 'int'),
        getSsaNodes((nodes) => ssaBeforeLoop = nodes[x]!),
        forEachWithNonVariable(expr('List<int?>'), [
          checkNotPromoted(x),
          getSsaNodes((nodes) => expect(nodes[x], isNot(ssaBeforeLoop))),
          x.write(expr('int?')).stmt,
        ]),
      ]);
    });

    test('forEach_bodyBegin() handles write captures in the loop', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        checkPromoted(x, 'int'),
        forEachWithNonVariable(expr('List<int?>'), [
          x.read.as_('int').stmt,
          checkNotPromoted(x),
          localFunction([
            x.write(expr('int?')).stmt,
          ]),
        ]),
      ]);
    });

    test('forEach_bodyBegin() writes to loop variable', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: false),
        checkAssigned(x, false),
        forEachWithVariableSet(x, expr('List<int?>'), [
          checkAssigned(x, true),
        ]),
        checkAssigned(x, false),
      ]);
    });

    test('forEach_bodyBegin() does not write capture loop variable', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: false),
        checkAssigned(x, false),
        forEachWithVariableSet(x, expr('List<int?>'), [
          checkAssigned(x, true),
          if_(x.read.notEq(nullLiteral), [checkPromoted(x, 'int')]),
        ]),
        checkAssigned(x, false),
      ]);
    });

    test('forEach_bodyBegin() pushes conservative join state', () {
      var h = Harness();
      var x = Var('x', 'int');
      h.run([
        declare(x, initialized: false),
        checkUnassigned(x, true),
        branchTarget((t) => forEachWithNonVariable(expr('List<int>'), [
              // Since a write to x occurs somewhere in the loop, x should no
              // longer be considered unassigned.
              checkUnassigned(x, false),
              break_(t), x.write(expr('int')).stmt,
            ])),
        // Even though the write to x is unreachable (since it occurs after a
        // break), x should still be considered "possibly assigned" because of
        // the conservative join done at the top of the loop.
        checkUnassigned(x, false),
      ]);
    });

    test('forEach_end() restores state before loop', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        forEachWithNonVariable(expr('List<int?>'), [
          x.read.as_('int').stmt,
          checkPromoted(x, 'int'),
        ]),
        checkNotPromoted(x),
      ]);
    });

    test('functionExpression_begin() cancels promotions of self-captured vars',
        () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        x.read.as_('int').stmt,
        y.read.as_('int').stmt,
        checkPromoted(x, 'int'),
        checkPromoted(y, 'int'),
        getSsaNodes((nodes) {
          expect(nodes[x], isNotNull);
          expect(nodes[y], isNotNull);
        }),
        localFunction([
          // x is unpromoted within the local function
          checkNotPromoted(x), checkPromoted(y, 'int'),
          getSsaNodes((nodes) {
            expect(nodes[x], isNull);
            expect(nodes[y], isNotNull);
          }),
          x.write(expr('int?')).stmt, x.read.as_('int').stmt,
        ]),
        // x is unpromoted after the local function too
        checkNotPromoted(x), checkPromoted(y, 'int'),
        getSsaNodes((nodes) {
          expect(nodes[x], isNull);
          expect(nodes[y], isNotNull);
        }),
      ]);
    });

    test('functionExpression_begin() cancels promotions of other-captured vars',
        () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      h.run([
        declare(x, initialized: true), declare(y, initialized: true),
        x.read.as_('int').stmt, y.read.as_('int').stmt,
        checkPromoted(x, 'int'), checkPromoted(y, 'int'),
        localFunction([
          // x is unpromoted within the local function, because the write
          // might have been captured by the time the local function executes.
          checkNotPromoted(x), checkPromoted(y, 'int'),
          // And any effort to promote x fails, because there is no way of
          // knowing when the captured write might occur.
          x.read.as_('int').stmt,
          checkNotPromoted(x), checkPromoted(y, 'int'),
        ]),
        // x is still promoted after the local function, though, because the
        // write hasn't been captured yet.
        checkPromoted(x, 'int'), checkPromoted(y, 'int'),
        localFunction([
          // x is unpromoted inside this local function too.
          checkNotPromoted(x), checkPromoted(y, 'int'),
          x.write(expr('int?')).stmt,
        ]),
        // And since the second local function captured x, it remains
        // unpromoted.
        checkNotPromoted(x), checkPromoted(y, 'int'),
      ]);
    });

    test('functionExpression_begin() cancels promotions of written vars', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      late SsaNode<Var, Type> ssaBeforeFunction;
      h.run([
        declare(x, initialized: true), declare(y, initialized: true),
        x.read.as_('int').stmt, y.read.as_('int').stmt,
        checkPromoted(x, 'int'),
        getSsaNodes((nodes) => ssaBeforeFunction = nodes[x]!),
        checkPromoted(y, 'int'),
        localFunction([
          // x is unpromoted within the local function, because the write
          // might have happened by the time the local function executes.
          checkNotPromoted(x),
          getSsaNodes((nodes) => expect(nodes[x], isNot(ssaBeforeFunction))),
          checkPromoted(y, 'int'),
          // But it can be re-promoted because the write isn't captured.
          x.read.as_('int').stmt,
          checkPromoted(x, 'int'), checkPromoted(y, 'int'),
        ]),
        // x is still promoted after the local function, though, because the
        // write hasn't occurred yet.
        checkPromoted(x, 'int'),
        getSsaNodes((nodes) => expect(nodes[x], same(ssaBeforeFunction))),
        checkPromoted(y, 'int'),
        x.write(expr('int?')).stmt,
        // x is unpromoted now.
        checkNotPromoted(x), checkPromoted(y, 'int'),
      ]);
    });

    test('functionExpression_begin() handles not-yet-seen variables', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        localFunction([]),
        // x is declared after the local function, so the local function
        // cannot possibly write to x.
        declare(x, initialized: true), x.read.as_('int').stmt,
        checkPromoted(x, 'int'), x.write(expr('Null')).stmt,
      ]);
    });

    test('functionExpression_begin() handles not-yet-seen write-captured vars',
        () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      h.run([
        declare(y, initialized: true),
        y.read.as_('int').stmt,
        getSsaNodes((nodes) => expect(nodes[x], null)),
        localFunction([
          getSsaNodes((nodes) => expect(nodes[x], isNot(nodes[y]))),
          x.read.as_('int').stmt,
          // Promotion should not occur, because x might be write-captured by
          // the time this code is reached.
          checkNotPromoted(x),
        ]),
        localFunction([
          declare(x, initialized: true),
          x.write(expr('Null')).stmt,
        ]),
      ]);
    });

    test(
        'functionExpression_end does not propagate "definitely unassigned" '
        'data', () {
      var h = Harness();
      var x = Var('x', 'int');
      h.run([
        declare(x, initialized: false),
        checkUnassigned(x, true),
        localFunction([
          // The function expression could be called at any time, so x might
          // be assigned now.
          checkUnassigned(x, false),
        ]),
        // But now that we are back outside the function expression, we once
        // again know that x is unassigned.
        checkUnassigned(x, true),
        x.write(expr('int')).stmt,
        checkUnassigned(x, false),
      ]);
    });

    test('handleBreak handles deep nesting', () {
      var h = Harness();
      h.run([
        branchTarget((t) => while_(booleanLiteral(true), [
              if_(expr('bool'), [
                if_(expr('bool'), [
                  break_(t),
                ]),
              ]),
              return_(),
              checkReachable(false),
            ])),
        checkReachable(true),
      ]);
    });

    test('handleBreak handles mixed nesting', () {
      var h = Harness();
      h.run([
        branchTarget((t) => while_(booleanLiteral(true), [
              if_(expr('bool'), [
                if_(expr('bool'), [
                  break_(t),
                ]),
                break_(t),
              ]),
              break_(t),
              checkReachable(false),
            ])),
        checkReachable(true),
      ]);
    });

    test('handleContinue handles deep nesting', () {
      var h = Harness();
      h.run([
        branchTarget((t) => do_([
              if_(expr('bool'), [
                if_(expr('bool'), [
                  continue_(t),
                ]),
              ]),
              return_(),
              checkReachable(false),
            ], checkReachable(true).thenExpr(booleanLiteral(true)))),
        checkReachable(false),
      ]);
    });

    test('handleContinue handles mixed nesting', () {
      var h = Harness();
      h.run([
        branchTarget((t) => do_([
              if_(expr('bool'), [
                if_(expr('bool'), [
                  continue_(t),
                ]),
                continue_(t),
              ]),
              continue_(t),
              checkReachable(false),
            ], checkReachable(true).thenExpr(booleanLiteral(true)))),
        checkReachable(false),
      ]);
    });

    test('ifNullExpression allows ensure guarding', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read
            .ifNull(block([
              checkReachable(true),
              x.write(expr('int')).stmt,
              checkPromoted(x, 'int'),
            ]).thenExpr(expr('int?')))
            .thenStmt(block([
              checkReachable(true),
              checkPromoted(x, 'int'),
            ]))
            .stmt,
      ]);
    });

    test('ifNullExpression allows promotion of tested var', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read
            .ifNull(block([
              checkReachable(true),
              x.read.as_('int').stmt,
              checkPromoted(x, 'int'),
            ]).thenExpr(expr('int?')))
            .thenStmt(block([
              checkReachable(true),
              checkPromoted(x, 'int'),
            ]))
            .stmt,
      ]);
    });

    test('ifNullExpression discards promotions unrelated to tested expr', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        expr('int?')
            .ifNull(block([
              checkReachable(true),
              x.read.as_('int').stmt,
              checkPromoted(x, 'int'),
            ]).thenExpr(expr('int?')))
            .thenStmt(block([
              checkReachable(true),
              checkNotPromoted(x),
            ]))
            .stmt,
      ]);
    });

    test('ifNullExpression does not detect when RHS is unreachable', () {
      var h = Harness();
      h.run([
        expr('int')
            .ifNull(checkReachable(true).thenExpr(expr('int')))
            .thenStmt(checkReachable(true))
            .stmt,
      ]);
    });

    test('ifNullExpression determines reachability correctly for `Null` type',
        () {
      var h = Harness();
      h.run([
        expr('Null')
            .ifNull(checkReachable(true).thenExpr(expr('Null')))
            .thenStmt(checkReachable(true))
            .stmt,
      ]);
    });

    test('ifStatement with early exit promotes in unreachable code', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        return_(),
        checkReachable(false),
        if_(x.read.eq(nullLiteral), [
          return_(),
        ]),
        checkReachable(false),
        checkPromoted(x, 'int'),
      ]);
    });

    test('ifStatement_end(false) keeps else branch if then branch exits', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        if_(x.read.eq(nullLiteral), [
          return_(),
        ]),
        checkPromoted(x, 'int'),
      ]);
    });

    test(
        'ifStatement_end() discards non-matching expression info from joined '
        'branches', () {
      var h = Harness();
      var w = Var('w', 'Object');
      var x = Var('x', 'bool');
      var y = Var('y', 'bool');
      var z = Var('z', 'bool');
      late SsaNode<Var, Type> xSsaNodeBeforeIf;
      h.run([
        declare(w, initialized: true),
        declare(x, initialized: true),
        declare(y, initialized: true),
        declare(z, initialized: true),
        x.write(w.read.is_('int')).stmt,
        getSsaNodes((nodes) {
          xSsaNodeBeforeIf = nodes[x]!;
          expect(xSsaNodeBeforeIf.expressionInfo, isNotNull);
        }),
        if_(expr('bool'), [
          y.write(w.read.is_('String')).stmt,
        ], [
          z.write(w.read.is_('bool')).stmt,
        ]),
        getSsaNodes((nodes) {
          expect(nodes[x], same(xSsaNodeBeforeIf));
          expect(nodes[y]!.expressionInfo, isNull);
          expect(nodes[z]!.expressionInfo, isNull);
        }),
      ]);
    });

    test(
        'ifStatement_end() ignores non-matching SSA info from "then" path if '
        'unreachable', () {
      var h = Harness();
      var x = Var('x', 'Object');
      late SsaNode<Var, Type> xSsaNodeBeforeIf;
      h.run([
        declare(x, initialized: true),
        getSsaNodes((nodes) {
          xSsaNodeBeforeIf = nodes[x]!;
        }),
        if_(expr('bool'), [
          x.write(expr('Object')).stmt,
          return_(),
        ]),
        getSsaNodes((nodes) {
          expect(nodes[x], same(xSsaNodeBeforeIf));
        }),
      ]);
    });

    test(
        'ifStatement_end() ignores non-matching SSA info from "else" path if '
        'unreachable', () {
      var h = Harness();
      var x = Var('x', 'Object');
      late SsaNode<Var, Type> xSsaNodeBeforeIf;
      h.run([
        declare(x, initialized: true),
        getSsaNodes((nodes) {
          xSsaNodeBeforeIf = nodes[x]!;
        }),
        if_(expr('bool'), [], [
          x.write(expr('Object')).stmt,
          return_(),
        ]),
        getSsaNodes((nodes) {
          expect(nodes[x], same(xSsaNodeBeforeIf));
        }),
      ]);
    });

    test('initialize() promotes when not final', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declareInitialized(x, expr('int')),
        checkPromoted(x, 'int'),
      ]);
    });

    test('initialize() does not promote when final', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declareInitialized(x, expr('int'), isFinal: true),
        checkNotPromoted(x),
      ]);
    });

    test('initialize() stores expressionInfo when not late', () {
      var h = Harness();
      var x = Var('x', 'Object');
      var y = Var('y', 'int?');
      late ExpressionInfo<Var, Type> writtenValueInfo;
      h.run([
        declareInitialized(
            x,
            y.read.eq(nullLiteral).getExpressionInfo((info) {
              expect(info, isNotNull);
              writtenValueInfo = info!;
            })),
        getSsaNodes((nodes) {
          expect(nodes[x]!.expressionInfo, same(writtenValueInfo));
        }),
      ]);
    });

    test('initialize() does not store expressionInfo when late', () {
      var h = Harness();
      var x = Var('x', 'Object');
      var y = Var('y', 'int?');
      h.run([
        declareInitialized(x, y.read.eq(nullLiteral), isLate: true),
        getSsaNodes((nodes) {
          expect(nodes[x]!.expressionInfo, isNull);
        }),
      ]);
    });

    test('initialize() does not store expressionInfo for trivial expressions',
        () {
      var h = Harness();
      var x = Var('x', 'Object');
      var y = Var('y', 'int?');
      h.run([
        declare(y, initialized: true),
        localFunction([
          y.write(expr('int?')).stmt,
        ]),
        declareInitialized(
            x,
            // `y == null` is a trivial expression because y has been write
            // captured.
            y.read
                .eq(nullLiteral)
                .getExpressionInfo((info) => expect(info, isNotNull))),
        getSsaNodes((nodes) {
          expect(nodes[x]!.expressionInfo, isNull);
        }),
      ]);
    });

    void _checkIs(String declaredType, String tryPromoteType,
        String? expectedPromotedTypeThen, String? expectedPromotedTypeElse,
        {bool inverted = false}) {
      var h = Harness();
      var x = Var('x', declaredType);
      late SsaNode<Var, Type> ssaBeforePromotion;
      h.run([
        declare(x, initialized: true),
        getSsaNodes((nodes) => ssaBeforePromotion = nodes[x]!),
        if_(x.read.is_(tryPromoteType, isInverted: inverted), [
          checkReachable(true),
          checkPromoted(x, expectedPromotedTypeThen),
          getSsaNodes((nodes) => expect(nodes[x], same(ssaBeforePromotion))),
        ], [
          checkReachable(true),
          checkPromoted(x, expectedPromotedTypeElse),
          getSsaNodes((nodes) => expect(nodes[x], same(ssaBeforePromotion))),
        ])
      ]);
    }

    test('isExpression_end promotes to a subtype', () {
      _checkIs('int?', 'int', 'int', 'Never?');
    });

    test('isExpression_end promotes to a subtype, inverted', () {
      _checkIs('int?', 'int', 'Never?', 'int', inverted: true);
    });

    test('isExpression_end does not promote to a supertype', () {
      _checkIs('int', 'int?', null, null);
    });

    test('isExpression_end does not promote to a supertype, inverted', () {
      _checkIs('int', 'int?', null, null, inverted: true);
    });

    test('isExpression_end does not promote to an unrelated type', () {
      _checkIs('int', 'String', null, null);
    });

    test('isExpression_end does not promote to an unrelated type, inverted',
        () {
      _checkIs('int', 'String', null, null, inverted: true);
    });

    test('isExpression_end does nothing if applied to a non-variable', () {
      var h = Harness();
      h.run([
        if_(expr('Null').is_('int'), [
          checkReachable(true),
        ], [
          checkReachable(true),
        ]),
      ]);
    });

    test('isExpression_end does nothing if applied to a non-variable, inverted',
        () {
      var h = Harness();
      h.run([
        if_(expr('Null').isNot('int'), [
          checkReachable(true),
        ], [
          checkReachable(true),
        ]),
      ]);
    });

    test('isExpression_end() does not promote write-captured vars', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        if_(x.read.is_('int'), [
          checkPromoted(x, 'int'),
        ]),
        localFunction([
          x.write(expr('int?')).stmt,
        ]),
        if_(x.read.is_('int'), [
          checkNotPromoted(x),
        ]),
      ]);
    });

    test('isExpression_end() handles not-yet-seen variables', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        if_(x.read.is_('int'), [
          checkPromoted(x, 'int'),
        ]),
        declare(x, initialized: true),
        localFunction([
          x.write(expr('Null')).stmt,
        ]),
      ]);
    });

    test('isExpression_end() does not set reachability for `this`', () {
      var h = Harness()
        ..addSubtype('Never', 'C', true)
        ..addFactor('C', 'Never', 'C');
      h.run([
        if_(this_('C').is_('Never'), [
          checkReachable(true),
        ], [
          checkReachable(true),
        ]),
      ]);
    });

    group('isExpression_end() does not set reachability for property gets', () {
      test('on a variable', () {
        var h = Harness();
        var x = Var('x', 'C');
        h.run([
          declare(x, initialized: true),
          if_(x.read.propertyGet('f').is_('Never'), [
            checkReachable(true),
          ], [
            checkReachable(true),
          ]),
        ]);
      });

      test('on an arbitrary expression', () {
        var h = Harness();
        h.run([
          if_(expr('C').propertyGet('f').is_('Never'), [
            checkReachable(true),
          ], [
            checkReachable(true),
          ]),
        ]);
      });

      test('on explicit this', () {
        var h = Harness();
        h.run([
          if_(this_('C').propertyGet('f').is_('Never'), [
            checkReachable(true),
          ], [
            checkReachable(true),
          ]),
        ]);
      });

      test('on implicit this/super', () {
        var h = Harness();
        h.run([
          if_(thisOrSuperPropertyGet('f').is_('Never'), [
            checkReachable(true),
          ], [
            checkReachable(true),
          ]),
        ]);
      });
    });

    test('labeledBlock without break', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        if_(x.read.isNot('int'), [
          labeled(return_()),
        ]),
        checkPromoted(x, 'int'),
      ]);
    });

    test('labeledBlock with break joins', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        if_(x.read.isNot('int'), [
          branchTarget((t) => labeled(block([
                if_(expr('bool'), [
                  break_(t),
                ]),
                return_(),
              ]))),
        ]),
        checkNotPromoted(x),
      ]);
    });

    test('logicalBinaryOp_rightBegin(isAnd: true) promotes in RHS', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read
            .notEq(nullLiteral)
            .and(checkPromoted(x, 'int').thenExpr(expr('bool')))
            .stmt,
      ]);
    });

    test('logicalBinaryOp_rightEnd(isAnd: true) keeps promotions from RHS', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        if_(expr('bool').and(x.read.notEq(nullLiteral)), [
          checkPromoted(x, 'int'),
        ]),
      ]);
    });

    test('logicalBinaryOp_rightEnd(isAnd: false) keeps promotions from RHS',
        () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        if_(expr('bool').or(x.read.eq(nullLiteral)), [], [
          checkPromoted(x, 'int'),
        ]),
      ]);
    });

    test('logicalBinaryOp_rightBegin(isAnd: false) promotes in RHS', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read
            .eq(nullLiteral)
            .or(checkPromoted(x, 'int').thenExpr(expr('bool')))
            .stmt,
      ]);
    });

    test('logicalBinaryOp(isAnd: true) joins promotions', () {
      // if (x != null && y != null) {
      //   promotes x and y
      // }
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        if_(x.read.notEq(nullLiteral).and(y.read.notEq(nullLiteral)), [
          checkPromoted(x, 'int'),
          checkPromoted(y, 'int'),
        ]),
      ]);
    });

    test('logicalBinaryOp(isAnd: false) joins promotions', () {
      // if (x == null || y == null) {} else {
      //   promotes x and y
      // }
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        if_(x.read.eq(nullLiteral).or(y.read.eq(nullLiteral)), [], [
          checkPromoted(x, 'int'),
          checkPromoted(y, 'int'),
        ]),
      ]);
    });

    test('logicalNot_end() inverts a condition', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        if_(x.read.eq(nullLiteral).not, [
          checkPromoted(x, 'int'),
        ], [
          checkNotPromoted(x),
        ]),
      ]);
    });

    test('logicalNot_end() handles null literals', () {
      var h = Harness();
      h.run([
        // `!null` would be a compile error, but we need to make sure we don't
        // crash.
        if_(nullLiteral.not, [], []),
      ]);
    });

    test('nonNullAssert_end(x) promotes', () {
      var h = Harness();
      var x = Var('x', 'int?');
      late SsaNode<Var, Type> ssaBeforePromotion;
      h.run([
        declare(x, initialized: true),
        getSsaNodes((nodes) => ssaBeforePromotion = nodes[x]!),
        x.read.nonNullAssert.stmt,
        checkPromoted(x, 'int'),
        getSsaNodes((nodes) => expect(nodes[x], same(ssaBeforePromotion))),
      ]);
    });

    test('nullAwareAccess temporarily promotes', () {
      var h = Harness();
      var x = Var('x', 'int?');
      late SsaNode<Var, Type> ssaBeforePromotion;
      h.run([
        declare(x, initialized: true),
        getSsaNodes((nodes) => ssaBeforePromotion = nodes[x]!),
        x.read
            .nullAwareAccess(block([
              checkReachable(true),
              checkPromoted(x, 'int'),
              getSsaNodes(
                  (nodes) => expect(nodes[x], same(ssaBeforePromotion))),
            ]).thenExpr(expr('Null')))
            .stmt,
        checkNotPromoted(x),
        getSsaNodes((nodes) => expect(nodes[x], same(ssaBeforePromotion))),
      ]);
    });

    test('nullAwareAccess does not promote the target of a cascade', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read
            .nullAwareAccess(
                block([
                  checkReachable(true),
                  checkNotPromoted(x),
                ]).thenExpr(expr('Null')),
                isCascaded: true)
            .stmt,
      ]);
    });

    test('nullAwareAccess preserves demotions', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        expr('int')
            .nullAwareAccess(block([
              checkReachable(true),
              checkPromoted(x, 'int'),
            ]).thenExpr(x.write(expr('int?'))).thenStmt(checkNotPromoted(x)))
            .stmt,
        checkNotPromoted(x),
      ]);
    });

    test('nullAwareAccess_end ignores shorting if target is non-nullable', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        expr('int')
            .nullAwareAccess(block([
              checkReachable(true),
              x.read.as_('int').stmt,
              checkPromoted(x, 'int'),
            ]).thenExpr(expr('Null')))
            .stmt,
        // Since the null-shorting path was reachable, promotion of `x` should
        // be cancelled.
        checkNotPromoted(x),
      ]);
    });

    test('parenthesizedExpression preserves promotion behaviors', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        if_(
            x.read.parenthesized.notEq(nullLiteral.parenthesized).parenthesized,
            [
              checkPromoted(x, 'int'),
            ]),
      ]);
    });

    test('promote promotes to a subtype and sets type of interest', () {
      var h = Harness();
      var x = Var('x', 'num?');
      h.run([
        declare(x, initialized: true),
        checkNotPromoted(x),
        x.read.as_('num').stmt,
        checkPromoted(x, 'num'),
        // Check that it's a type of interest by promoting and de-promoting.
        if_(x.read.is_('int'), [
          checkPromoted(x, 'int'),
          x.write(expr('num')).stmt,
          checkPromoted(x, 'num'),
        ]),
      ]);
    });

    test('promote does not promote to a non-subtype', () {
      var h = Harness();
      var x = Var('x', 'num?');
      h.run([
        declare(x, initialized: true),
        checkNotPromoted(x),
        x.read.as_('String').stmt,
        checkNotPromoted(x),
      ]);
    });

    test('promote does not promote if variable is write-captured', () {
      var h = Harness();
      var x = Var('x', 'num?');
      h.run([
        declare(x, initialized: true),
        checkNotPromoted(x),
        localFunction([
          x.write(expr('num')).stmt,
        ]),
        x.read.as_('num').stmt,
        checkNotPromoted(x),
      ]);
    });

    test('promotedType handles not-yet-seen variables', () {
      // Note: this is needed for error recovery in the analyzer.
      var h = Harness();
      var x = Var('x', 'int');
      h.run([
        checkNotPromoted(x),
        declare(x, initialized: true),
      ]);
    });

    test('switchStatement_beginCase(false) restores previous promotions', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        switch_(
            expr('Null'),
            [
              case_([
                checkPromoted(x, 'int'),
                x.write(expr('int?')).stmt,
                checkNotPromoted(x),
              ]),
              case_([
                checkPromoted(x, 'int'),
                x.write(expr('int?')).stmt,
                checkNotPromoted(x),
              ]),
            ],
            isExhaustive: false),
      ]);
    });

    test('switchStatement_beginCase(false) does not un-promote', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        switch_(
            expr('Null'),
            [
              case_([
                checkPromoted(x, 'int'),
                x.write(expr('int?')).stmt,
                checkNotPromoted(x),
              ])
            ],
            isExhaustive: false),
      ]);
    });

    test('switchStatement_beginCase(false) handles write captures in cases',
        () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        switch_(
            expr('Null'),
            [
              case_([
                checkPromoted(x, 'int'),
                localFunction([
                  x.write(expr('int?')).stmt,
                ]),
                checkNotPromoted(x),
              ]),
            ],
            isExhaustive: false),
      ]);
    });

    test('switchStatement_beginCase(true) un-promotes', () {
      var h = Harness();
      var x = Var('x', 'int?');
      late SsaNode<Var, Type> ssaBeforeSwitch;
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        switch_(
            expr('Null').thenStmt(block([
              checkPromoted(x, 'int'),
              getSsaNodes((nodes) => ssaBeforeSwitch = nodes[x]!),
            ])),
            [
              case_([
                checkNotPromoted(x),
                getSsaNodes(
                    (nodes) => expect(nodes[x], isNot(ssaBeforeSwitch))),
                x.write(expr('int?')).stmt,
                checkNotPromoted(x),
              ], hasLabel: true),
            ],
            isExhaustive: false),
      ]);
    });

    test('switchStatement_beginCase(true) handles write captures in cases', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        switch_(
            expr('Null'),
            [
              case_([
                x.read.as_('int').stmt,
                checkNotPromoted(x),
                localFunction([
                  x.write(expr('int?')).stmt,
                ]),
                checkNotPromoted(x),
              ], hasLabel: true),
            ],
            isExhaustive: false),
      ]);
    });

    test('switchStatement_end(false) joins break and default', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      var z = Var('z', 'int?');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        declare(z, initialized: true),
        y.read.as_('int').stmt,
        z.read.as_('int').stmt,
        branchTarget((t) => switch_(
            expr('Null'),
            [
              case_([
                x.read.as_('int').stmt,
                y.write(expr('int?')).stmt,
                break_(t),
              ]),
            ],
            isExhaustive: false)),
        checkNotPromoted(x),
        checkNotPromoted(y),
        checkPromoted(z, 'int'),
      ]);
    });

    test('switchStatement_end(true) joins breaks', () {
      var h = Harness();
      var w = Var('w', 'int?');
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      var z = Var('z', 'int?');
      h.run([
        declare(w, initialized: true),
        declare(x, initialized: true),
        declare(y, initialized: true),
        declare(z, initialized: true),
        x.read.as_('int').stmt,
        y.read.as_('int').stmt,
        z.read.as_('int').stmt,
        branchTarget((t) => switch_(
            expr('Null'),
            [
              case_([
                w.read.as_('int').stmt,
                y.read.as_('int').stmt,
                x.write(expr('int?')).stmt,
                break_(t),
              ]),
              case_([
                w.read.as_('int').stmt,
                x.read.as_('int').stmt,
                y.write(expr('int?')).stmt,
                break_(t),
              ]),
            ],
            isExhaustive: true)),
        checkPromoted(w, 'int'),
        checkNotPromoted(x),
        checkNotPromoted(y),
        checkPromoted(z, 'int'),
      ]);
    });

    test('switchStatement_end(true) allows fall-through of last case', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        branchTarget((t) => switch_(
            expr('Null'),
            [
              case_([
                x.read.as_('int').stmt,
                break_(t),
              ]),
              case_([]),
            ],
            isExhaustive: true)),
        checkNotPromoted(x),
      ]);
    });

    test('tryCatchStatement_bodyEnd() restores pre-try state', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        y.read.as_('int').stmt,
        tryCatch([
          x.read.as_('int').stmt,
          checkPromoted(x, 'int'),
          checkPromoted(y, 'int'),
        ], [
          catch_(body: [
            checkNotPromoted(x),
            checkPromoted(y, 'int'),
          ])
        ]),
      ]);
    });

    test('tryCatchStatement_bodyEnd() un-promotes variables assigned in body',
        () {
      var h = Harness();
      var x = Var('x', 'int?');
      late SsaNode<Var, Type> ssaAfterTry;
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        checkPromoted(x, 'int'),
        tryCatch([
          x.write(expr('int?')).stmt,
          x.read.as_('int').stmt,
          checkPromoted(x, 'int'),
          getSsaNodes((nodes) => ssaAfterTry = nodes[x]!),
        ], [
          catch_(body: [
            checkNotPromoted(x),
            getSsaNodes((nodes) => expect(nodes[x], isNot(ssaAfterTry))),
          ]),
        ]),
      ]);
    });

    test('tryCatchStatement_bodyEnd() preserves write captures in body', () {
      // Note: it's not necessary for the write capture to survive to the end of
      // the try body, because an exception could occur at any time.  We check
      // this by putting an exit in the try body.
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        checkPromoted(x, 'int'),
        tryCatch([
          localFunction([
            x.write(expr('int?')).stmt,
          ]),
          return_(),
        ], [
          catch_(body: [
            x.read.as_('int').stmt,
            checkNotPromoted(x),
          ])
        ]),
      ]);
    });

    test('tryCatchStatement_catchBegin() restores previous post-body state',
        () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        tryCatch([], [
          catch_(body: [
            x.read.as_('int').stmt,
            checkPromoted(x, 'int'),
          ]),
          catch_(body: [
            checkNotPromoted(x),
          ]),
        ]),
      ]);
    });

    test('tryCatchStatement_catchBegin() initializes vars', () {
      var h = Harness();
      var e = Var('e', 'int');
      var st = Var('st', 'StackTrace');
      h.run([
        tryCatch([], [
          catch_(exception: e, stackTrace: st, body: [
            checkAssigned(e, true),
            checkAssigned(st, true),
          ]),
        ]),
      ]);
    });

    test('tryCatchStatement_catchEnd() joins catch state with after-try state',
        () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      var z = Var('z', 'int?');
      h.run([
        declare(x, initialized: true), declare(y, initialized: true),
        declare(z, initialized: true),
        tryCatch([
          x.read.as_('int').stmt,
          y.read.as_('int').stmt,
        ], [
          catch_(body: [
            x.read.as_('int').stmt,
            z.read.as_('int').stmt,
          ]),
        ]),
        // Only x should be promoted, because it's the only variable
        // promoted in both the try body and the catch handler.
        checkPromoted(x, 'int'), checkNotPromoted(y), checkNotPromoted(z),
      ]);
    });

    test('tryCatchStatement_catchEnd() joins catch states', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      var z = Var('z', 'int?');
      h.run([
        declare(x, initialized: true), declare(y, initialized: true),
        declare(z, initialized: true),
        tryCatch([
          return_(),
        ], [
          catch_(body: [
            x.read.as_('int').stmt,
            y.read.as_('int').stmt,
          ]),
          catch_(body: [
            x.read.as_('int').stmt,
            z.read.as_('int').stmt,
          ]),
        ]),
        // Only x should be promoted, because it's the only variable promoted
        // in both catch handlers.
        checkPromoted(x, 'int'), checkNotPromoted(y), checkNotPromoted(z),
      ]);
    });

    test('tryFinallyStatement_finallyBegin() restores pre-try state', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        y.read.as_('int').stmt,
        tryFinally([
          x.read.as_('int').stmt,
          checkPromoted(x, 'int'),
          checkPromoted(y, 'int'),
        ], [
          checkNotPromoted(x),
          checkPromoted(y, 'int'),
        ]),
      ]);
    });

    test(
        'tryFinallyStatement_finallyBegin() un-promotes variables assigned in '
        'body', () {
      var h = Harness();
      var x = Var('x', 'int?');
      late SsaNode<Var, Type> ssaAtStartOfTry;
      late SsaNode<Var, Type> ssaAfterTry;
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        checkPromoted(x, 'int'),
        tryFinally([
          getSsaNodes((nodes) => ssaAtStartOfTry = nodes[x]!),
          x.write(expr('int?')).stmt,
          x.read.as_('int').stmt,
          checkPromoted(x, 'int'),
          getSsaNodes((nodes) => ssaAfterTry = nodes[x]!),
        ], [
          checkNotPromoted(x),
          // The SSA node for X should be different from what it was at any time
          // during the try block, because there is no telling at what point an
          // exception might have occurred.
          getSsaNodes((nodes) {
            expect(nodes[x], isNot(ssaAtStartOfTry));
            expect(nodes[x], isNot(ssaAfterTry));
          }),
        ]),
      ]);
    });

    test('tryFinallyStatement_finallyBegin() preserves write captures in body',
        () {
      // Note: it's not necessary for the write capture to survive to the end of
      // the try body, because an exception could occur at any time.  We check
      // this by putting an exit in the try body.
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        tryFinally([
          localFunction([
            x.write(expr('int?')).stmt,
          ]),
          return_(),
        ], [
          x.read.as_('int').stmt,
          checkNotPromoted(x),
        ]),
      ]);
    });

    test('tryFinallyStatement_end() restores promotions from try body', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      h.run([
        declare(x, initialized: true), declare(y, initialized: true),
        tryFinally([
          x.read.as_('int').stmt,
          checkPromoted(x, 'int'),
        ], [
          checkNotPromoted(x),
          y.read.as_('int').stmt,
          checkPromoted(y, 'int'),
        ]),
        // Both x and y should now be promoted.
        checkPromoted(x, 'int'), checkPromoted(y, 'int'),
      ]);
    });

    test(
        'tryFinallyStatement_end() does not restore try body promotions for '
        'variables assigned in finally', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      late SsaNode<Var, Type> xSsaAtEndOfFinally;
      late SsaNode<Var, Type> ySsaAtEndOfFinally;
      h.run([
        declare(x, initialized: true), declare(y, initialized: true),
        tryFinally([
          x.read.as_('int').stmt,
          checkPromoted(x, 'int'),
        ], [
          checkNotPromoted(x),
          x.write(expr('int?')).stmt,
          y.write(expr('int?')).stmt,
          y.read.as_('int').stmt,
          checkPromoted(y, 'int'),
          getSsaNodes((nodes) {
            xSsaAtEndOfFinally = nodes[x]!;
            ySsaAtEndOfFinally = nodes[y]!;
          }),
        ]),
        // x should not be re-promoted, because it might have been assigned a
        // non-promoted value in the "finally" block.  But y's promotion still
        // stands, because y was promoted in the finally block.
        checkNotPromoted(x), checkPromoted(y, 'int'),
        // Both x and y should have the same SSA nodes they had at the end of
        // the finally block, since the finally block is guaranteed to have
        // executed.
        getSsaNodes((nodes) {
          expect(nodes[x], same(xSsaAtEndOfFinally));
          expect(nodes[y], same(ySsaAtEndOfFinally));
        }),
      ]);
    });

    group('allowLocalBooleanVarsToPromote', () {
      test(
          'tryFinallyStatement_end() restores SSA nodes from try block when it'
          'is sound to do so', () {
        var h = Harness();
        var x = Var('x', 'int?');
        var y = Var('y', 'int?');
        late SsaNode<Var, Type> xSsaAtEndOfTry;
        late SsaNode<Var, Type> ySsaAtEndOfTry;
        late SsaNode<Var, Type> xSsaAtEndOfFinally;
        late SsaNode<Var, Type> ySsaAtEndOfFinally;
        h.run([
          declare(x, initialized: true), declare(y, initialized: true),
          tryFinally([
            x.write(expr('int?')).stmt,
            y.write(expr('int?')).stmt,
            getSsaNodes((nodes) {
              xSsaAtEndOfTry = nodes[x]!;
              ySsaAtEndOfTry = nodes[y]!;
            }),
          ], [
            if_(expr('bool'), [
              x.write(expr('int?')).stmt,
            ]),
            if_(expr('bool'), [
              y.write(expr('int?')).stmt,
              return_(),
            ]),
            getSsaNodes((nodes) {
              xSsaAtEndOfFinally = nodes[x]!;
              ySsaAtEndOfFinally = nodes[y]!;
              expect(xSsaAtEndOfFinally, isNot(same(xSsaAtEndOfTry)));
              expect(ySsaAtEndOfFinally, isNot(same(ySsaAtEndOfTry)));
            }),
          ]),
          // x's SSA node should still match what it was at the end of the
          // finally block, because it might have been written to.  But y
          // can't have been written to, because once we reach here, we know
          // that the finally block completed normally, and the write to y
          // always leads to the explicit return.  So y's SSA node should be
          // restored back to match that from the end of the try block.
          getSsaNodes((nodes) {
            expect(nodes[x], same(xSsaAtEndOfFinally));
            expect(nodes[y], same(ySsaAtEndOfTry));
          }),
        ]);
      });

      test(
          'tryFinallyStatement_end() sets unreachable if end of try block '
          'unreachable', () {
        var h = Harness();
        h.run([
          tryFinally([
            return_(),
            checkReachable(false),
          ], [
            checkReachable(true),
          ]),
          checkReachable(false),
        ]);
      });

      test(
          'tryFinallyStatement_end() sets unreachable if end of finally block '
          'unreachable', () {
        var h = Harness();
        h.run([
          tryFinally([
            checkReachable(true),
          ], [
            return_(),
            checkReachable(false),
          ]),
          checkReachable(false),
        ]);
      });

      test(
          'tryFinallyStatement_end() handles a variable declared only in the '
          'try block', () {
        var h = Harness();
        var x = Var('x', 'int?');
        h.run([
          tryFinally([
            declare(x, initialized: true),
          ], []),
        ]);
      });

      test(
          'tryFinallyStatement_end() handles a variable declared only in the '
          'finally block', () {
        var h = Harness();
        var x = Var('x', 'int?');
        h.run([
          tryFinally([], [
            declare(x, initialized: true),
          ]),
        ]);
      });

      test(
          'tryFinallyStatement_end() handles a variable that was write '
          'captured in the try block', () {
        var h = Harness();
        var x = Var('x', 'int?');
        h.run([
          declare(x, initialized: true),
          tryFinally([
            localFunction([
              x.write(expr('int?')).stmt,
            ]),
          ], []),
          if_(x.read.notEq(nullLiteral), [
            checkNotPromoted(x),
          ]),
        ]);
      });

      test(
          'tryFinallyStatement_end() handles a variable that was write '
          'captured in the finally block', () {
        var h = Harness();
        var x = Var('x', 'int?');
        h.run([
          declare(x, initialized: true),
          tryFinally([], [
            localFunction([
              x.write(expr('int?')).stmt,
            ]),
          ]),
          if_(x.read.notEq(nullLiteral), [
            checkNotPromoted(x),
          ]),
        ]);
      });

      test(
          'tryFinallyStatement_end() handles a variable that was promoted in '
          'the try block and write captured in the finally block', () {
        var h = Harness();
        var x = Var('x', 'int?');
        h.run([
          declare(x, initialized: true),
          tryFinally([
            if_(x.read.eq(nullLiteral), [
              return_(),
            ]),
            checkPromoted(x, 'int'),
          ], [
            localFunction([
              x.write(expr('int?')).stmt,
            ]),
          ]),
          // The capture in the `finally` cancels old promotions and prevents
          // future promotions.
          checkNotPromoted(x),
          if_(x.read.notEq(nullLiteral), [
            checkNotPromoted(x),
          ]),
        ]);
      });

      test(
          'tryFinallyStatement_end() keeps promotions from both try and '
          'finally blocks when there is no write in the finally block', () {
        var h = Harness();
        var x = Var('x', 'Object');
        h.run([
          declare(x, initialized: true),
          tryFinally([
            if_(x.read.is_('num', isInverted: true), [
              return_(),
            ]),
            checkPromoted(x, 'num'),
          ], [
            if_(x.read.is_('int', isInverted: true), [
              return_(),
            ]),
          ]),
          // The promotion chain now contains both `num` and `int`.
          checkPromoted(x, 'int'),
          x.write(expr('num')).stmt,
          checkPromoted(x, 'num'),
        ]);
      });

      test(
          'tryFinallyStatement_end() keeps promotions from the finally block '
          'when there is a write in the finally block', () {
        var h = Harness();
        var x = Var('x', 'Object');
        h.run([
          declare(x, initialized: true),
          tryFinally([
            if_(x.read.is_('String', isInverted: true), [
              return_(),
            ]),
            checkPromoted(x, 'String'),
          ], [
            x.write(expr('Object')).stmt,
            if_(x.read.is_('int', isInverted: true), [
              return_(),
            ]),
          ]),
          checkPromoted(x, 'int'),
        ]);
      });

      test(
          'tryFinallyStatement_end() keeps tests from both the try and finally '
          'blocks', () {
        var h = Harness();
        var x = Var('x', 'Object');
        h.run([
          declare(x, initialized: true),
          tryFinally([
            if_(x.read.is_('String', isInverted: true), []),
            checkNotPromoted(x),
          ], [
            if_(x.read.is_('int', isInverted: true), []),
            checkNotPromoted(x),
          ]),
          checkNotPromoted(x),
          if_(expr('bool'), [
            x.write(expr('String')).stmt,
            checkPromoted(x, 'String'),
          ], [
            x.write(expr('int')).stmt,
            checkPromoted(x, 'int'),
          ]),
        ]);
      });

      test(
          'tryFinallyStatement_end() handles variables not definitely assigned '
          'in either the try or finally block', () {
        var h = Harness();
        var x = Var('x', 'Object');
        h.run([
          declare(x, initialized: false),
          checkAssigned(x, false),
          tryFinally([
            if_(expr('bool'), [
              x.write(expr('Object')).stmt,
            ]),
            checkAssigned(x, false),
          ], [
            if_(expr('bool'), [
              x.write(expr('Object')).stmt,
            ]),
            checkAssigned(x, false),
          ]),
          checkAssigned(x, false),
        ]);
      });

      test(
          'tryFinallyStatement_end() handles variables definitely assigned in '
          'the try block', () {
        var h = Harness();
        var x = Var('x', 'Object');
        h.run([
          declare(x, initialized: false),
          checkAssigned(x, false),
          tryFinally([
            x.write(expr('Object')).stmt,
            checkAssigned(x, true),
          ], [
            if_(expr('bool'), [
              x.write(expr('Object')).stmt,
            ]),
            checkAssigned(x, false),
          ]),
          checkAssigned(x, true),
        ]);
      });

      test(
          'tryFinallyStatement_end() handles variables definitely assigned in '
          'the finally block', () {
        var h = Harness();
        var x = Var('x', 'Object');
        h.run([
          declare(x, initialized: false),
          checkAssigned(x, false),
          tryFinally([
            if_(expr('bool'), [
              x.write(expr('Object')).stmt,
            ]),
            checkAssigned(x, false),
          ], [
            x.write(expr('Object')).stmt,
            checkAssigned(x, true),
          ]),
          checkAssigned(x, true),
        ]);
      });

      test(
          'tryFinallyStatement_end() handles variables definitely unassigned '
          'in both the try and finally blocks', () {
        var h = Harness();
        var x = Var('x', 'Object');
        h.run([
          declare(x, initialized: false),
          checkUnassigned(x, true),
          tryFinally([
            checkUnassigned(x, true),
          ], [
            checkUnassigned(x, true),
          ]),
          checkUnassigned(x, true),
        ]);
      });

      test(
          'tryFinallyStatement_end() handles variables definitely unassigned '
          'in the try but not the finally block', () {
        var h = Harness();
        var x = Var('x', 'Object');
        h.run([
          declare(x, initialized: false),
          checkUnassigned(x, true),
          tryFinally([
            checkUnassigned(x, true),
          ], [
            if_(expr('bool'), [
              x.write(expr('Object')).stmt,
            ]),
            checkUnassigned(x, false),
          ]),
          checkUnassigned(x, false),
        ]);
      });

      test(
          'tryFinallyStatement_end() handles variables definitely unassigned '
          'in the finally but not the try block', () {
        var h = Harness();
        var x = Var('x', 'Object');
        h.run([
          declare(x, initialized: false),
          checkUnassigned(x, true),
          tryFinally([
            if_(expr('bool'), [
              x.write(expr('Object')).stmt,
            ]),
            checkUnassigned(x, false),
          ], [
            checkUnassigned(x, false),
          ]),
          checkUnassigned(x, false),
        ]);
      });
    });

    test('variableRead() restores promotions from previous write()', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      var z = Var('z', 'bool');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        declare(z, initialized: true),
        // Create a variable that promotes x if its value is true, and y if its
        // value is false.
        z
            .write(x.read.notEq(nullLiteral).conditional(
                booleanLiteral(true),
                y.read.notEq(nullLiteral).conditional(
                    booleanLiteral(false), throw_(expr('Object')))))
            .stmt,
        checkNotPromoted(x),
        checkNotPromoted(y),
        // Simply reading the variable shouldn't promote anything.
        z.read.stmt,
        checkNotPromoted(x),
        checkNotPromoted(y),
        // But reading it in an "if" condition should promote.
        if_(z.read, [
          checkPromoted(x, 'int'),
          checkNotPromoted(y),
        ], [
          checkNotPromoted(x),
          checkPromoted(y, 'int'),
        ]),
      ]);
    });

    test('variableRead() restores promotions from previous initialization', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      var z = Var('z', 'bool');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        // Create a variable that promotes x if its value is true, and y if its
        // value is false.
        declareInitialized(
            z,
            x.read.notEq(nullLiteral).conditional(
                booleanLiteral(true),
                y.read.notEq(nullLiteral).conditional(
                    booleanLiteral(false), throw_(expr('Object'))))),
        checkNotPromoted(x),
        checkNotPromoted(y),
        // Simply reading the variable shouldn't promote anything.
        z.read.stmt,
        checkNotPromoted(x),
        checkNotPromoted(y),
        // But reading it in an "if" condition should promote.
        if_(z.read, [
          checkPromoted(x, 'int'),
          checkNotPromoted(y),
        ], [
          checkNotPromoted(x),
          checkPromoted(y, 'int'),
        ]),
      ]);
    });

    test('variableRead() rebases old promotions', () {
      var h = Harness();
      var w = Var('w', 'int?');
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      var z = Var('z', 'bool');
      h.run([
        declare(w, initialized: true),
        declare(x, initialized: true),
        declare(y, initialized: true),
        declare(z, initialized: true),
        // Create a variable that promotes x if its value is true, and y if its
        // value is false.
        z
            .write(x.read.notEq(nullLiteral).conditional(
                booleanLiteral(true),
                y.read.notEq(nullLiteral).conditional(
                    booleanLiteral(false), throw_(expr('Object')))))
            .stmt,
        checkNotPromoted(w),
        checkNotPromoted(x),
        checkNotPromoted(y),
        w.read.nonNullAssert.stmt,
        checkPromoted(w, 'int'),
        // Reading the value of z in an "if" condition should promote x or y,
        // and keep the promotion of w.
        if_(z.read, [
          checkPromoted(w, 'int'),
          checkPromoted(x, 'int'),
          checkNotPromoted(y),
        ], [
          checkPromoted(w, 'int'),
          checkNotPromoted(x),
          checkPromoted(y, 'int'),
        ]),
      ]);
    });

    test("variableRead() doesn't restore the notion of whether a value is null",
        () {
      // Note: we have the available infrastructure to do this if we want, but
      // we think it will give an inconsistent feel because comparisons like
      // `if (i == null)` *don't* promote.
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        y.write(nullLiteral).stmt,
        checkNotPromoted(x),
        checkNotPromoted(y),
        if_(x.read.eq(y.read), [
          checkNotPromoted(x),
          checkNotPromoted(y),
        ], [
          // Even though x != y and y is known to contain the value `null`, we
          // don't promote x.
          checkNotPromoted(x),
          checkNotPromoted(y),
        ]),
      ]);
    });

    test('whileStatement_conditionBegin() un-promotes', () {
      var h = Harness();
      var x = Var('x', 'int?');
      late SsaNode<Var, Type> ssaBeforeLoop;
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        checkPromoted(x, 'int'),
        getSsaNodes((nodes) => ssaBeforeLoop = nodes[x]!),
        while_(
            block([
              checkNotPromoted(x),
              getSsaNodes((nodes) => expect(nodes[x], isNot(ssaBeforeLoop))),
            ]).thenExpr(expr('bool')),
            [
              x.write(expr('Null')).stmt,
            ]),
      ]);
    });

    test('whileStatement_conditionBegin() handles write captures in the loop',
        () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        checkPromoted(x, 'int'),
        while_(
            block([
              x.read.as_('int').stmt,
              checkNotPromoted(x),
              localFunction([
                x.write(expr('int?')).stmt,
              ]),
            ]).thenExpr(expr('bool')),
            []),
      ]);
    });

    test('whileStatement_conditionBegin() handles not-yet-seen variables', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      h.run([
        declare(y, initialized: true),
        y.read.as_('int').stmt,
        while_(declare(x, initialized: true).thenExpr(expr('bool')), [
          x.write(expr('Null')).stmt,
        ]),
      ]);
    });

    test('whileStatement_bodyBegin() promotes', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        while_(x.read.notEq(nullLiteral), [
          checkPromoted(x, 'int'),
        ]),
      ]);
    });

    test('whileStatement_end() joins break and condition-false states', () {
      // To test that the states are properly joined, we have three variables:
      // x, y, and z.  We promote x and y in the break path, and x and z in the
      // condition-false path.  After the loop, only x should be promoted.
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      var z = Var('z', 'int?');
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        declare(z, initialized: true),
        branchTarget(
            (t) => while_(x.read.eq(nullLiteral).or(z.read.eq(nullLiteral)), [
                  if_(expr('bool'), [
                    x.read.as_('int').stmt,
                    y.read.as_('int').stmt,
                    break_(t),
                  ]),
                ])),
        checkPromoted(x, 'int'),
        checkNotPromoted(y),
        checkNotPromoted(z),
      ]);
    });

    test('whileStatement_end() with break updates Ssa of modified vars', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('x', 'int?');
      late SsaNode<Var, Type> xSsaInsideLoop;
      late SsaNode<Var, Type> ySsaInsideLoop;
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        branchTarget((t) => while_(expr('bool'), [
              x.write(expr('int?')).stmt,
              if_(expr('bool'), [break_(t)]),
              getSsaNodes((nodes) {
                xSsaInsideLoop = nodes[x]!;
                ySsaInsideLoop = nodes[y]!;
              }),
            ])),
        getSsaNodes((nodes) {
          // x's Ssa should have been changed because of the join at the end of
          // the loop.  y's should not, since it retains the value it had prior
          // to the loop.
          expect(nodes[x], isNot(xSsaInsideLoop));
          expect(nodes[y], same(ySsaInsideLoop));
        }),
      ]);
    });

    test(
        'whileStatement_end() with break updates Ssa of modified vars when '
        'types were tested', () {
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('x', 'int?');
      late SsaNode<Var, Type> xSsaInsideLoop;
      late SsaNode<Var, Type> ySsaInsideLoop;
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        branchTarget((t) => while_(expr('bool'), [
              x.write(expr('int?')).stmt,
              if_(expr('bool'), [break_(t)]),
              if_(x.read.is_('int'), []),
              getSsaNodes((nodes) {
                xSsaInsideLoop = nodes[x]!;
                ySsaInsideLoop = nodes[y]!;
              }),
            ])),
        getSsaNodes((nodes) {
          // x's Ssa should have been changed because of the join at the end of
          // the loop.  y's should not, since it retains the value it had prior
          // to the loop.
          expect(nodes[x], isNot(xSsaInsideLoop));
          expect(nodes[y], same(ySsaInsideLoop));
        }),
      ]);
    });

    test('write() de-promotes and updates Ssa of a promoted variable', () {
      var h = Harness();
      var x = Var('x', 'Object');
      var y = Var('y', 'int?');
      late SsaNode<Var, Type> ssaBeforeWrite;
      late ExpressionInfo<Var, Type> writtenValueInfo;
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        x.read.as_('int').stmt,
        checkPromoted(x, 'int'),
        getSsaNodes((nodes) => ssaBeforeWrite = nodes[x]!),
        x
            .write(y.read.eq(nullLiteral).getExpressionInfo((info) {
              expect(info, isNotNull);
              writtenValueInfo = info!;
            }))
            .stmt,
        checkNotPromoted(x),
        getSsaNodes((nodes) {
          expect(nodes[x], isNot(ssaBeforeWrite));
          expect(nodes[x]!.expressionInfo, same(writtenValueInfo));
        }),
      ]);
    });

    test('write() updates Ssa', () {
      var h = Harness();
      var x = Var('x', 'Object');
      var y = Var('y', 'int?');
      late SsaNode<Var, Type> ssaBeforeWrite;
      late ExpressionInfo<Var, Type> writtenValueInfo;
      h.run([
        declare(x, initialized: true),
        getSsaNodes((nodes) => ssaBeforeWrite = nodes[x]!),
        x
            .write(y.read.eq(nullLiteral).getExpressionInfo((info) {
              expect(info, isNotNull);
              writtenValueInfo = info!;
            }))
            .stmt,
        getSsaNodes((nodes) {
          expect(nodes[x], isNot(ssaBeforeWrite));
          expect(nodes[x]!.expressionInfo, same(writtenValueInfo));
        }),
      ]);
    });

    test('write() does not copy Ssa from one variable to another', () {
      // We could do so, and it would enable us to promote in slightly more
      // situations, e.g.:
      //   bool b = x != null;
      //   if (b) { /* x promoted here */ }
      //   var tmp = x;
      //   x = ...;
      //   if (b) { /* x not promoted here */ }
      //   x = tmp;
      //   if (b) { /* x promoted again */ }
      // But there are a lot of corner cases to test and it's not clear how much
      // the benefit will be, so for now we're not doing it.
      var h = Harness();
      var x = Var('x', 'int?');
      var y = Var('y', 'int?');
      late SsaNode<Var, Type> xSsaBeforeWrite;
      late SsaNode<Var, Type> ySsa;
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        getSsaNodes((nodes) {
          xSsaBeforeWrite = nodes[x]!;
          ySsa = nodes[y]!;
        }),
        x.write(y.read).stmt,
        getSsaNodes((nodes) {
          expect(nodes[x], isNot(xSsaBeforeWrite));
          expect(nodes[x], isNot(ySsa));
        }),
      ]);
    });

    test('write() does not store expressionInfo for trivial expressions', () {
      var h = Harness();
      var x = Var('x', 'Object');
      var y = Var('y', 'int?');
      late SsaNode<Var, Type> ssaBeforeWrite;
      h.run([
        declare(x, initialized: true),
        declare(y, initialized: true),
        localFunction([
          y.write(expr('int?')).stmt,
        ]),
        getSsaNodes((nodes) => ssaBeforeWrite = nodes[x]!),
        // `y == null` is a trivial expression because y has been write
        // captured.
        x
            .write(y.read
                .eq(nullLiteral)
                .getExpressionInfo((info) => expect(info, isNotNull)))
            .stmt,
        getSsaNodes((nodes) {
          expect(nodes[x], isNot(ssaBeforeWrite));
          expect(nodes[x]!.expressionInfo, isNull);
        }),
      ]);
    });

    test('write() permits expression to be null', () {
      var h = Harness();
      var x = Var('x', 'Object');
      late SsaNode<Var, Type> ssaBeforeWrite;
      h.run([
        declare(x, initialized: true),
        getSsaNodes((nodes) => ssaBeforeWrite = nodes[x]!),
        x.write(null).stmt,
        getSsaNodes((nodes) {
          expect(nodes[x], isNot(ssaBeforeWrite));
          expect(nodes[x]!.expressionInfo, isNull);
        }),
      ]);
    });

    test('Infinite loop does not implicitly assign variables', () {
      var h = Harness();
      var x = Var('x', 'int');
      h.run([
        declare(x, initialized: false),
        while_(booleanLiteral(true), [
          x.write(expr('Null')).stmt,
        ]),
        checkAssigned(x, false),
      ]);
    });

    test('If(false) does not discard promotions', () {
      var h = Harness();
      var x = Var('x', 'Object');
      h.run([
        declare(x, initialized: true),
        x.read.as_('int').stmt,
        checkPromoted(x, 'int'),
        if_(booleanLiteral(false), [
          checkPromoted(x, 'int'),
        ]),
      ]);
    });

    test('Promotions do not occur when a variable is write-captured', () {
      var h = Harness();
      var x = Var('x', 'Object');
      h.run([
        declare(x, initialized: true),
        localFunction([
          x.write(expr('Object')).stmt,
        ]),
        getSsaNodes((nodes) => expect(nodes[x], isNull)),
        x.read.as_('int').stmt,
        checkNotPromoted(x),
        getSsaNodes((nodes) => expect(nodes[x], isNull)),
      ]);
    });

    test('Promotion cancellation of write-captured vars survives join', () {
      var h = Harness();
      var x = Var('x', 'Object');
      h.run([
        declare(x, initialized: true),
        if_(expr('bool'), [
          localFunction([
            x.write(expr('Object')).stmt,
          ]),
        ], [
          // Promotion should work here because the write capture is in the
          // other branch.
          x.read.as_('int').stmt, checkPromoted(x, 'int'),
        ]),
        // But the promotion should be cancelled now, after the join.
        checkNotPromoted(x),
        // And further attempts to promote should fail due to the write capture.
        x.read.as_('int').stmt, checkNotPromoted(x),
      ]);
    });
  });

  group('Reachability', () {
    test('initial state', () {
      expect(Reachability.initial.parent, isNull);
      expect(Reachability.initial.locallyReachable, true);
      expect(Reachability.initial.overallReachable, true);
    });

    test('split', () {
      var reachableSplit = Reachability.initial.split();
      expect(reachableSplit.parent, same(Reachability.initial));
      expect(reachableSplit.overallReachable, true);
      expect(reachableSplit.locallyReachable, true);
      var unreachable = reachableSplit.setUnreachable();
      var unreachableSplit = unreachable.split();
      expect(unreachableSplit.parent, same(unreachable));
      expect(unreachableSplit.overallReachable, false);
      expect(unreachableSplit.locallyReachable, true);
    });

    test('unsplit', () {
      var base = Reachability.initial.split();
      var reachableSplit = base.split();
      var reachableSplitUnsplit = reachableSplit.unsplit();
      expect(reachableSplitUnsplit.parent, same(base.parent));
      expect(reachableSplitUnsplit.overallReachable, true);
      expect(reachableSplitUnsplit.locallyReachable, true);
      var reachableSplitUnreachable = reachableSplit.setUnreachable();
      var reachableSplitUnreachableUnsplit =
          reachableSplitUnreachable.unsplit();
      expect(reachableSplitUnreachableUnsplit.parent, same(base.parent));
      expect(reachableSplitUnreachableUnsplit.overallReachable, false);
      expect(reachableSplitUnreachableUnsplit.locallyReachable, false);
      var unreachable = base.setUnreachable();
      var unreachableSplit = unreachable.split();
      var unreachableSplitUnsplit = unreachableSplit.unsplit();
      expect(unreachableSplitUnsplit, same(unreachable));
      var unreachableSplitUnreachable = unreachableSplit.setUnreachable();
      var unreachableSplitUnreachableUnsplit =
          unreachableSplitUnreachable.unsplit();
      expect(unreachableSplitUnreachableUnsplit, same(unreachable));
    });

    test('setUnreachable', () {
      var reachable = Reachability.initial.split();
      var unreachable = reachable.setUnreachable();
      expect(unreachable.parent, same(reachable.parent));
      expect(unreachable.locallyReachable, false);
      expect(unreachable.overallReachable, false);
      expect(unreachable.setUnreachable(), same(unreachable));
      var provisionallyReachable = unreachable.split();
      var provisionallyUnreachable = provisionallyReachable.setUnreachable();
      expect(
          provisionallyUnreachable.parent, same(provisionallyReachable.parent));
      expect(provisionallyUnreachable.locallyReachable, false);
      expect(provisionallyUnreachable.overallReachable, false);
      expect(provisionallyUnreachable.setUnreachable(),
          same(provisionallyUnreachable));
    });

    test('restrict', () {
      var previous = Reachability.initial.split();
      var reachable = previous.split();
      var unreachable = reachable.setUnreachable();
      expect(Reachability.restrict(reachable, reachable), same(reachable));
      expect(Reachability.restrict(reachable, unreachable), same(unreachable));
      expect(Reachability.restrict(unreachable, reachable), same(unreachable));
      expect(
          Reachability.restrict(unreachable, unreachable), same(unreachable));
    });

    test('rebaseForward', () {
      var previous = Reachability.initial;
      var reachable = previous.split();
      var reachable2 = previous.split();
      var unreachable = reachable.setUnreachable();
      var unreachablePrevious = previous.setUnreachable();
      var reachable3 = unreachablePrevious.split();
      expect(reachable.rebaseForward(reachable), same(reachable));
      expect(reachable.rebaseForward(reachable2), same(reachable2));
      expect(reachable.rebaseForward(unreachable), same(unreachable));
      expect(unreachable.rebaseForward(reachable).parent, same(previous));
      expect(unreachable.rebaseForward(reachable).locallyReachable, false);
      expect(unreachable.rebaseForward(unreachable), same(unreachable));
      expect(reachable.rebaseForward(unreachablePrevious),
          same(unreachablePrevious));
      expect(
          unreachablePrevious.rebaseForward(reachable).parent, same(previous));
      expect(
          unreachablePrevious.rebaseForward(reachable).locallyReachable, false);
      expect(reachable.rebaseForward(reachable3), same(reachable3));
      expect(reachable3.rebaseForward(reachable).parent, same(previous));
      expect(reachable3.rebaseForward(reachable).locallyReachable, false);
    });

    test('join', () {
      var previous = Reachability.initial.split();
      var reachable = previous.split();
      var unreachable = reachable.setUnreachable();
      expect(Reachability.join(reachable, reachable), same(reachable));
      expect(Reachability.join(reachable, unreachable), same(reachable));
      expect(Reachability.join(unreachable, reachable), same(reachable));
      expect(Reachability.join(unreachable, unreachable), same(unreachable));
    });

    test('commonAncestor', () {
      var parent1 = Reachability.initial;
      var parent2 = parent1.setUnreachable();
      var child1 = parent1.split();
      var child2 = parent1.split();
      var child3 = child1.split();
      var child4 = child2.split();
      expect(Reachability.commonAncestor(null, null), null);
      expect(Reachability.commonAncestor(null, parent1), null);
      expect(Reachability.commonAncestor(parent1, null), null);
      expect(Reachability.commonAncestor(null, child1), null);
      expect(Reachability.commonAncestor(child1, null), null);
      expect(Reachability.commonAncestor(null, child3), null);
      expect(Reachability.commonAncestor(child3, null), null);
      expect(Reachability.commonAncestor(parent1, parent1), same(parent1));
      expect(Reachability.commonAncestor(parent1, parent2), null);
      expect(Reachability.commonAncestor(parent2, child1), null);
      expect(Reachability.commonAncestor(child1, parent2), null);
      expect(Reachability.commonAncestor(parent2, child3), null);
      expect(Reachability.commonAncestor(child3, parent2), null);
      expect(Reachability.commonAncestor(parent1, child1), same(parent1));
      expect(Reachability.commonAncestor(child1, parent1), same(parent1));
      expect(Reachability.commonAncestor(parent1, child3), same(parent1));
      expect(Reachability.commonAncestor(child3, parent1), same(parent1));
      expect(Reachability.commonAncestor(child1, child1), same(child1));
      expect(Reachability.commonAncestor(child1, child2), same(parent1));
      expect(Reachability.commonAncestor(child1, child3), same(child1));
      expect(Reachability.commonAncestor(child3, child1), same(child1));
      expect(Reachability.commonAncestor(child1, child4), same(parent1));
      expect(Reachability.commonAncestor(child4, child1), same(parent1));
      expect(Reachability.commonAncestor(child3, child3), same(child3));
      expect(Reachability.commonAncestor(child3, child4), same(parent1));
    });
  });

  group('State', () {
    var intVar = Var('x', 'int');
    var intQVar = Var('x', 'int?');
    var objectQVar = Var('x', 'Object?');
    var nullVar = Var('x', 'Null');
    group('setUnreachable', () {
      var unreachable =
          FlowModel<Var, Type>(Reachability.initial.setUnreachable());
      var reachable = FlowModel<Var, Type>(Reachability.initial);
      test('unchanged', () {
        expect(unreachable.setUnreachable(), same(unreachable));
      });

      test('changed', () {
        void _check(FlowModel<Var, Type> initial) {
          var s = initial.setUnreachable();
          expect(s, isNot(same(initial)));
          expect(s.reachable.overallReachable, false);
          expect(s.variableInfo, same(initial.variableInfo));
        }

        _check(reachable);
      });
    });

    test('split', () {
      var s1 = FlowModel<Var, Type>(Reachability.initial);
      var s2 = s1.split();
      expect(s2.reachable.parent, same(s1.reachable));
    });

    test('unsplit', () {
      var s1 = FlowModel<Var, Type>(Reachability.initial.split());
      var s2 = s1.unsplit();
      expect(s2.reachable, same(Reachability.initial));
    });

    group('unsplitTo', () {
      test('no change', () {
        var s1 = FlowModel<Var, Type>(Reachability.initial.split());
        var result = s1.unsplitTo(s1.reachable.parent!);
        expect(result, same(s1));
      });

      test('unsplit once, reachable', () {
        var s1 = FlowModel<Var, Type>(Reachability.initial.split());
        var s2 = s1.split();
        var result = s2.unsplitTo(s1.reachable.parent!);
        expect(result.reachable, same(s1.reachable));
      });

      test('unsplit once, unreachable', () {
        var s1 = FlowModel<Var, Type>(Reachability.initial.split());
        var s2 = s1.split().setUnreachable();
        var result = s2.unsplitTo(s1.reachable.parent!);
        expect(result.reachable.locallyReachable, false);
        expect(result.reachable.parent, same(s1.reachable.parent));
      });

      test('unsplit twice, reachable', () {
        var s1 = FlowModel<Var, Type>(Reachability.initial.split());
        var s2 = s1.split();
        var s3 = s2.split();
        var result = s3.unsplitTo(s1.reachable.parent!);
        expect(result.reachable, same(s1.reachable));
      });

      test('unsplit twice, top unreachable', () {
        var s1 = FlowModel<Var, Type>(Reachability.initial.split());
        var s2 = s1.split();
        var s3 = s2.split().setUnreachable();
        var result = s3.unsplitTo(s1.reachable.parent!);
        expect(result.reachable.locallyReachable, false);
        expect(result.reachable.parent, same(s1.reachable.parent));
      });

      test('unsplit twice, previous unreachable', () {
        var s1 = FlowModel<Var, Type>(Reachability.initial.split());
        var s2 = s1.split().setUnreachable();
        var s3 = s2.split();
        var result = s3.unsplitTo(s1.reachable.parent!);
        expect(result.reachable.locallyReachable, false);
        expect(result.reachable.parent, same(s1.reachable.parent));
      });
    });

    group('tryPromoteForTypeCheck', () {
      test('unpromoted -> unchanged (same)', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial);
        var s2 = s1._tryPromoteForTypeCheck(h, intVar, 'int').ifTrue;
        expect(s2, same(s1));
      });

      test('unpromoted -> unchanged (supertype)', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial);
        var s2 = s1._tryPromoteForTypeCheck(h, intVar, 'Object').ifTrue;
        expect(s2, same(s1));
      });

      test('unpromoted -> unchanged (unrelated)', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial);
        var s2 = s1._tryPromoteForTypeCheck(h, intVar, 'String').ifTrue;
        expect(s2, same(s1));
      });

      test('unpromoted -> subtype', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial);
        var s2 = s1._tryPromoteForTypeCheck(h, intQVar, 'int').ifTrue;
        expect(s2.reachable.overallReachable, true);
        expect(s2.variableInfo, {
          intQVar: _matchVariableModel(chain: ['int'], ofInterest: ['int'])
        });
      });

      test('promoted -> unchanged (same)', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            ._tryPromoteForTypeCheck(h, objectQVar, 'int')
            .ifTrue;
        var s2 = s1._tryPromoteForTypeCheck(h, objectQVar, 'int').ifTrue;
        expect(s2, same(s1));
      });

      test('promoted -> unchanged (supertype)', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            ._tryPromoteForTypeCheck(h, objectQVar, 'int')
            .ifTrue;
        var s2 = s1._tryPromoteForTypeCheck(h, objectQVar, 'Object').ifTrue;
        expect(s2, same(s1));
      });

      test('promoted -> unchanged (unrelated)', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            ._tryPromoteForTypeCheck(h, objectQVar, 'int')
            .ifTrue;
        var s2 = s1._tryPromoteForTypeCheck(h, objectQVar, 'String').ifTrue;
        expect(s2, same(s1));
      });

      test('promoted -> subtype', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            ._tryPromoteForTypeCheck(h, objectQVar, 'int?')
            .ifTrue;
        var s2 = s1._tryPromoteForTypeCheck(h, objectQVar, 'int').ifTrue;
        expect(s2.reachable.overallReachable, true);
        expect(s2.variableInfo, {
          objectQVar: _matchVariableModel(
              chain: ['int?', 'int'], ofInterest: ['int?', 'int'])
        });
      });
    });

    group('write', () {
      var objectQVar = Var('x', 'Object?');

      test('without declaration', () {
        // This should not happen in valid code, but test that we don't crash.
        var h = Harness();
        var s = FlowModel<Var, Type>(Reachability.initial).write(
            null, objectQVar, Type('Object?'), new SsaNode<Var, Type>(null), h);
        expect(s.variableInfo[objectQVar], isNull);
      });

      test('unchanged', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            .declare(objectQVar, true);
        var s2 = s1.write(
            null, objectQVar, Type('Object?'), new SsaNode<Var, Type>(null), h);
        expect(s2, isNot(same(s1)));
        expect(s2.reachable, same(s1.reachable));
        expect(
            s2.infoFor(objectQVar),
            _matchVariableModel(
                chain: null,
                ofInterest: isEmpty,
                assigned: true,
                unassigned: false));
      });

      test('marks as assigned', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            .declare(objectQVar, false);
        var s2 = s1.write(
            null, objectQVar, Type('int?'), new SsaNode<Var, Type>(null), h);
        expect(s2.reachable.overallReachable, true);
        expect(
            s2.infoFor(objectQVar),
            _matchVariableModel(
                chain: null,
                ofInterest: isEmpty,
                assigned: true,
                unassigned: false));
      });

      test('un-promotes fully', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            .declare(objectQVar, true)
            ._tryPromoteForTypeCheck(h, objectQVar, 'int')
            .ifTrue;
        expect(s1.variableInfo, contains(objectQVar));
        var s2 = s1.write(_MockNonPromotionReason(), objectQVar, Type('int?'),
            new SsaNode<Var, Type>(null), h);
        expect(s2.reachable.overallReachable, true);
        expect(s2.variableInfo, {
          objectQVar: _matchVariableModel(
              chain: null,
              ofInterest: isEmpty,
              assigned: true,
              unassigned: false)
        });
      });

      test('un-promotes partially, when no exact match', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            .declare(objectQVar, true)
            ._tryPromoteForTypeCheck(h, objectQVar, 'num?')
            .ifTrue
            ._tryPromoteForTypeCheck(h, objectQVar, 'int')
            .ifTrue;
        expect(s1.variableInfo, {
          objectQVar: _matchVariableModel(
              chain: ['num?', 'int'],
              ofInterest: ['num?', 'int'],
              assigned: true,
              unassigned: false)
        });
        var s2 = s1.write(_MockNonPromotionReason(), objectQVar, Type('num'),
            new SsaNode<Var, Type>(null), h);
        expect(s2.reachable.overallReachable, true);
        expect(s2.variableInfo, {
          objectQVar: _matchVariableModel(
              chain: ['num?', 'num'],
              ofInterest: ['num?', 'int'],
              assigned: true,
              unassigned: false)
        });
      });

      test('un-promotes partially, when exact match', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            .declare(objectQVar, true)
            ._tryPromoteForTypeCheck(h, objectQVar, 'num?')
            .ifTrue
            ._tryPromoteForTypeCheck(h, objectQVar, 'num')
            .ifTrue
            ._tryPromoteForTypeCheck(h, objectQVar, 'int')
            .ifTrue;
        expect(s1.variableInfo, {
          objectQVar: _matchVariableModel(
              chain: ['num?', 'num', 'int'],
              ofInterest: ['num?', 'num', 'int'],
              assigned: true,
              unassigned: false)
        });
        var s2 = s1.write(_MockNonPromotionReason(), objectQVar, Type('num'),
            new SsaNode<Var, Type>(null), h);
        expect(s2.reachable.overallReachable, true);
        expect(s2.variableInfo, {
          objectQVar: _matchVariableModel(
              chain: ['num?', 'num'],
              ofInterest: ['num?', 'num', 'int'],
              assigned: true,
              unassigned: false)
        });
      });

      test('leaves promoted, when exact match', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            .declare(objectQVar, true)
            ._tryPromoteForTypeCheck(h, objectQVar, 'num?')
            .ifTrue
            ._tryPromoteForTypeCheck(h, objectQVar, 'num')
            .ifTrue;
        expect(s1.variableInfo, {
          objectQVar: _matchVariableModel(
              chain: ['num?', 'num'],
              ofInterest: ['num?', 'num'],
              assigned: true,
              unassigned: false)
        });
        var s2 = s1.write(
            null, objectQVar, Type('num'), new SsaNode<Var, Type>(null), h);
        expect(s2.reachable.overallReachable, true);
        expect(s2.variableInfo, isNot(same(s1.variableInfo)));
        expect(s2.variableInfo, {
          objectQVar: _matchVariableModel(
              chain: ['num?', 'num'],
              ofInterest: ['num?', 'num'],
              assigned: true,
              unassigned: false)
        });
      });

      test('leaves promoted, when writing a subtype', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            .declare(objectQVar, true)
            ._tryPromoteForTypeCheck(h, objectQVar, 'num?')
            .ifTrue
            ._tryPromoteForTypeCheck(h, objectQVar, 'num')
            .ifTrue;
        expect(s1.variableInfo, {
          objectQVar: _matchVariableModel(
              chain: ['num?', 'num'],
              ofInterest: ['num?', 'num'],
              assigned: true,
              unassigned: false)
        });
        var s2 = s1.write(
            null, objectQVar, Type('int'), new SsaNode<Var, Type>(null), h);
        expect(s2.reachable.overallReachable, true);
        expect(s2.variableInfo, isNot(same(s1.variableInfo)));
        expect(s2.variableInfo, {
          objectQVar: _matchVariableModel(
              chain: ['num?', 'num'],
              ofInterest: ['num?', 'num'],
              assigned: true,
              unassigned: false)
        });
      });

      group('Promotes to NonNull of a type of interest', () {
        test('when declared type', () {
          var h = Harness();
          var x = Var('x', 'int?');

          var s1 = FlowModel<Var, Type>(Reachability.initial).declare(x, true);
          expect(s1.variableInfo, {
            x: _matchVariableModel(chain: null),
          });

          var s2 =
              s1.write(null, x, Type('int'), new SsaNode<Var, Type>(null), h);
          expect(s2.variableInfo, {
            x: _matchVariableModel(chain: ['int']),
          });
        });

        test('when declared type, if write-captured', () {
          var h = Harness();
          var x = Var('x', 'int?');

          var s1 = FlowModel<Var, Type>(Reachability.initial).declare(x, true);
          expect(s1.variableInfo, {
            x: _matchVariableModel(chain: null),
          });

          var s2 = s1.conservativeJoin([], [x]);
          expect(s2.variableInfo, {
            x: _matchVariableModel(chain: null, writeCaptured: true),
          });

          // 'x' is write-captured, so not promoted
          var s3 =
              s2.write(null, x, Type('int'), new SsaNode<Var, Type>(null), h);
          expect(s3.variableInfo, {
            x: _matchVariableModel(chain: null, writeCaptured: true),
          });
        });

        test('when promoted', () {
          var h = Harness();
          var s1 = FlowModel<Var, Type>(Reachability.initial)
              .declare(objectQVar, true)
              ._tryPromoteForTypeCheck(h, objectQVar, 'int?')
              .ifTrue;
          expect(s1.variableInfo, {
            objectQVar: _matchVariableModel(
              chain: ['int?'],
              ofInterest: ['int?'],
            ),
          });
          var s2 = s1.write(
              null, objectQVar, Type('int'), new SsaNode<Var, Type>(null), h);
          expect(s2.variableInfo, {
            objectQVar: _matchVariableModel(
              chain: ['int?', 'int'],
              ofInterest: ['int?'],
            ),
          });
        });

        test('when not promoted', () {
          var h = Harness();
          var s1 = FlowModel<Var, Type>(Reachability.initial)
              .declare(objectQVar, true)
              ._tryPromoteForTypeCheck(h, objectQVar, 'int?')
              .ifFalse;
          expect(s1.variableInfo, {
            objectQVar: _matchVariableModel(
              chain: ['Object'],
              ofInterest: ['int?'],
            ),
          });
          var s2 = s1.write(
              null, objectQVar, Type('int'), new SsaNode<Var, Type>(null), h);
          expect(s2.variableInfo, {
            objectQVar: _matchVariableModel(
              chain: ['Object', 'int'],
              ofInterest: ['int?'],
            ),
          });
        });
      });

      test('Promotes to type of interest when not previously promoted', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            .declare(objectQVar, true)
            ._tryPromoteForTypeCheck(h, objectQVar, 'num?')
            .ifFalse;
        expect(s1.variableInfo, {
          objectQVar: _matchVariableModel(
            chain: ['Object'],
            ofInterest: ['num?'],
          ),
        });
        var s2 = s1.write(_MockNonPromotionReason(), objectQVar, Type('num?'),
            new SsaNode<Var, Type>(null), h);
        expect(s2.variableInfo, {
          objectQVar: _matchVariableModel(
            chain: ['num?'],
            ofInterest: ['num?'],
          ),
        });
      });

      test('Promotes to type of interest when previously promoted', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            .declare(objectQVar, true)
            ._tryPromoteForTypeCheck(h, objectQVar, 'num?')
            .ifTrue
            ._tryPromoteForTypeCheck(h, objectQVar, 'int?')
            .ifFalse;
        expect(s1.variableInfo, {
          objectQVar: _matchVariableModel(
            chain: ['num?', 'num'],
            ofInterest: ['num?', 'int?'],
          ),
        });
        var s2 = s1.write(_MockNonPromotionReason(), objectQVar, Type('int?'),
            new SsaNode<Var, Type>(null), h);
        expect(s2.variableInfo, {
          objectQVar: _matchVariableModel(
            chain: ['num?', 'int?'],
            ofInterest: ['num?', 'int?'],
          ),
        });
      });

      group('Multiple candidate types of interest', () {
        group('; choose most specific', () {
          late Harness h;

          setUp(() {
            h = Harness();

            // class A {}
            // class B extends A {}
            // class C extends B {}
            h.addSubtype('Object', 'A', false);
            h.addSubtype('Object', 'A?', false);
            h.addSubtype('Object', 'B?', false);
            h.addSubtype('A', 'Object', true);
            h.addSubtype('A', 'Object?', true);
            h.addSubtype('A', 'A?', true);
            h.addSubtype('A', 'B', false);
            h.addSubtype('A', 'B?', false);
            h.addSubtype('A?', 'Object', false);
            h.addSubtype('A?', 'Object?', true);
            h.addSubtype('A?', 'A', false);
            h.addSubtype('A?', 'B?', false);
            h.addSubtype('B', 'Object', true);
            h.addSubtype('B', 'A', true);
            h.addSubtype('B', 'A?', true);
            h.addSubtype('B', 'B?', true);
            h.addSubtype('B?', 'Object', false);
            h.addSubtype('B?', 'Object?', true);
            h.addSubtype('B?', 'A', false);
            h.addSubtype('B?', 'A?', true);
            h.addSubtype('B?', 'B', false);
            h.addSubtype('C', 'Object', true);
            h.addSubtype('C', 'A', true);
            h.addSubtype('C', 'A?', true);
            h.addSubtype('C', 'B', true);
            h.addSubtype('C', 'B?', true);

            h.addFactor('Object', 'A?', 'Object');
            h.addFactor('Object', 'B?', 'Object');
            h.addFactor('Object?', 'A', 'Object?');
            h.addFactor('Object?', 'A?', 'Object');
            h.addFactor('Object?', 'B?', 'Object');
          });

          test('; first', () {
            var x = Var('x', 'Object?');

            var s1 = FlowModel<Var, Type>(Reachability.initial)
                .declare(x, true)
                ._tryPromoteForTypeCheck(h, x, 'B?')
                .ifFalse
                ._tryPromoteForTypeCheck(h, x, 'A?')
                .ifFalse;
            expect(s1.variableInfo, {
              x: _matchVariableModel(
                chain: ['Object'],
                ofInterest: ['A?', 'B?'],
              ),
            });

            var s2 =
                s1.write(null, x, Type('C'), new SsaNode<Var, Type>(null), h);
            expect(s2.variableInfo, {
              x: _matchVariableModel(
                chain: ['Object', 'B'],
                ofInterest: ['A?', 'B?'],
              ),
            });
          });

          test('; second', () {
            var x = Var('x', 'Object?');

            var s1 = FlowModel<Var, Type>(Reachability.initial)
                .declare(x, true)
                ._tryPromoteForTypeCheck(h, x, 'A?')
                .ifFalse
                ._tryPromoteForTypeCheck(h, x, 'B?')
                .ifFalse;
            expect(s1.variableInfo, {
              x: _matchVariableModel(
                chain: ['Object'],
                ofInterest: ['A?', 'B?'],
              ),
            });

            var s2 =
                s1.write(null, x, Type('C'), new SsaNode<Var, Type>(null), h);
            expect(s2.variableInfo, {
              x: _matchVariableModel(
                chain: ['Object', 'B'],
                ofInterest: ['A?', 'B?'],
              ),
            });
          });

          test('; nullable and non-nullable', () {
            var x = Var('x', 'Object?');

            var s1 = FlowModel<Var, Type>(Reachability.initial)
                .declare(x, true)
                ._tryPromoteForTypeCheck(h, x, 'A')
                .ifFalse
                ._tryPromoteForTypeCheck(h, x, 'A?')
                .ifFalse;
            expect(s1.variableInfo, {
              x: _matchVariableModel(
                chain: ['Object'],
                ofInterest: ['A', 'A?'],
              ),
            });

            var s2 =
                s1.write(null, x, Type('B'), new SsaNode<Var, Type>(null), h);
            expect(s2.variableInfo, {
              x: _matchVariableModel(
                chain: ['Object', 'A'],
                ofInterest: ['A', 'A?'],
              ),
            });
          });
        });

        group('; ambiguous', () {
          test('; no promotion', () {
            var h = Harness();
            var s1 = FlowModel<Var, Type>(Reachability.initial)
                .declare(objectQVar, true)
                ._tryPromoteForTypeCheck(h, objectQVar, 'num?')
                .ifFalse
                ._tryPromoteForTypeCheck(h, objectQVar, 'num*')
                .ifFalse;
            expect(s1.variableInfo, {
              objectQVar: _matchVariableModel(
                chain: ['Object'],
                ofInterest: ['num?', 'num*'],
              ),
            });
            var s2 = s1.write(
                null, objectQVar, Type('int'), new SsaNode<Var, Type>(null), h);
            // It's ambiguous whether to promote to num? or num*, so we don't
            // promote.
            expect(s2, isNot(same(s1)));
            expect(s2.variableInfo, {
              objectQVar: _matchVariableModel(
                chain: ['Object'],
                ofInterest: ['num?', 'num*'],
              ),
            });
          });
        });

        test('exact match', () {
          var h = Harness();
          var s1 = FlowModel<Var, Type>(Reachability.initial)
              .declare(objectQVar, true)
              ._tryPromoteForTypeCheck(h, objectQVar, 'num?')
              .ifFalse
              ._tryPromoteForTypeCheck(h, objectQVar, 'num*')
              .ifFalse;
          expect(s1.variableInfo, {
            objectQVar: _matchVariableModel(
              chain: ['Object'],
              ofInterest: ['num?', 'num*'],
            ),
          });
          var s2 = s1.write(_MockNonPromotionReason(), objectQVar, Type('num?'),
              new SsaNode<Var, Type>(null), h);
          // It's ambiguous whether to promote to num? or num*, but since the
          // written type is exactly num?, we use that.
          expect(s2.variableInfo, {
            objectQVar: _matchVariableModel(
              chain: ['num?'],
              ofInterest: ['num?', 'num*'],
            ),
          });
        });
      });
    });

    group('demotion, to NonNull', () {
      test('when promoted via test', () {
        var x = Var('x', 'Object?');

        var h = Harness();

        var s1 = FlowModel<Var, Type>(Reachability.initial)
            .declare(x, true)
            ._tryPromoteForTypeCheck(h, x, 'num?')
            .ifTrue
            ._tryPromoteForTypeCheck(h, x, 'int?')
            .ifTrue;
        expect(s1.variableInfo, {
          x: _matchVariableModel(
            chain: ['num?', 'int?'],
            ofInterest: ['num?', 'int?'],
          ),
        });

        var s2 = s1.write(_MockNonPromotionReason(), x, Type('double'),
            new SsaNode<Var, Type>(null), h);
        expect(s2.variableInfo, {
          x: _matchVariableModel(
            chain: ['num?', 'num'],
            ofInterest: ['num?', 'int?'],
          ),
        });
      });
    });

    group('declare', () {
      var objectQVar = Var('x', 'Object?');

      test('initialized', () {
        var s = FlowModel<Var, Type>(Reachability.initial)
            .declare(objectQVar, true);
        expect(s.variableInfo, {
          objectQVar: _matchVariableModel(assigned: true, unassigned: false),
        });
      });

      test('not initialized', () {
        var s = FlowModel<Var, Type>(Reachability.initial)
            .declare(objectQVar, false);
        expect(s.variableInfo, {
          objectQVar: _matchVariableModel(assigned: false, unassigned: true),
        });
      });
    });

    group('markNonNullable', () {
      test('unpromoted -> unchanged', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial);
        var s2 = s1._tryMarkNonNullable(h, intVar).ifTrue;
        expect(s2, same(s1));
      });

      test('unpromoted -> promoted', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial);
        var s2 = s1._tryMarkNonNullable(h, intQVar).ifTrue;
        expect(s2.reachable.overallReachable, true);
        expect(s2.infoFor(intQVar),
            _matchVariableModel(chain: ['int'], ofInterest: []));
      });

      test('promoted -> unchanged', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            ._tryPromoteForTypeCheck(h, objectQVar, 'int')
            .ifTrue;
        var s2 = s1._tryMarkNonNullable(h, objectQVar).ifTrue;
        expect(s2, same(s1));
      });

      test('promoted -> re-promoted', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            ._tryPromoteForTypeCheck(h, objectQVar, 'int?')
            .ifTrue;
        var s2 = s1._tryMarkNonNullable(h, objectQVar).ifTrue;
        expect(s2.reachable.overallReachable, true);
        expect(s2.variableInfo, {
          objectQVar:
              _matchVariableModel(chain: ['int?', 'int'], ofInterest: ['int?'])
        });
      });

      test('promote to Never', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial);
        var s2 = s1._tryMarkNonNullable(h, nullVar).ifTrue;
        expect(s2.reachable.overallReachable, false);
        expect(s2.infoFor(nullVar),
            _matchVariableModel(chain: ['Never'], ofInterest: []));
      });
    });

    group('conservativeJoin', () {
      test('unchanged', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            .declare(intQVar, true)
            ._tryPromoteForTypeCheck(h, objectQVar, 'int')
            .ifTrue;
        var s2 = s1.conservativeJoin([intQVar], []);
        expect(s2, isNot(same(s1)));
        expect(s2.reachable, same(s1.reachable));
        expect(s2.variableInfo, {
          objectQVar: _matchVariableModel(chain: ['int'], ofInterest: ['int']),
          intQVar: _matchVariableModel(chain: null, ofInterest: [])
        });
      });

      test('written', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            ._tryPromoteForTypeCheck(h, objectQVar, 'int')
            .ifTrue
            ._tryPromoteForTypeCheck(h, intQVar, 'int')
            .ifTrue;
        var s2 = s1.conservativeJoin([intQVar], []);
        expect(s2.reachable.overallReachable, true);
        expect(s2.variableInfo, {
          objectQVar: _matchVariableModel(chain: ['int'], ofInterest: ['int']),
          intQVar: _matchVariableModel(chain: null, ofInterest: ['int'])
        });
      });

      test('write captured', () {
        var h = Harness();
        var s1 = FlowModel<Var, Type>(Reachability.initial)
            ._tryPromoteForTypeCheck(h, objectQVar, 'int')
            .ifTrue
            ._tryPromoteForTypeCheck(h, intQVar, 'int')
            .ifTrue;
        var s2 = s1.conservativeJoin([], [intQVar]);
        expect(s2.reachable.overallReachable, true);
        expect(s2.variableInfo, {
          objectQVar: _matchVariableModel(chain: ['int'], ofInterest: ['int']),
          intQVar: _matchVariableModel(
              chain: null, ofInterest: isEmpty, unassigned: false)
        });
      });
    });

    group('rebaseForward', () {
      test('reachability', () {
        var h = Harness();
        var reachable = FlowModel<Var, Type>(Reachability.initial);
        var unreachable = reachable.setUnreachable();
        expect(reachable.rebaseForward(h, reachable), same(reachable));
        expect(reachable.rebaseForward(h, unreachable), same(unreachable));
        expect(
            unreachable.rebaseForward(h, reachable).reachable.overallReachable,
            false);
        expect(unreachable.rebaseForward(h, reachable).variableInfo,
            same(unreachable.variableInfo));
        expect(unreachable.rebaseForward(h, unreachable), same(unreachable));
      });

      test('assignments', () {
        var h = Harness();
        var a = Var('a', 'int');
        var b = Var('b', 'int');
        var c = Var('c', 'int');
        var d = Var('d', 'int');
        var s0 = FlowModel<Var, Type>(Reachability.initial)
            .declare(a, false)
            .declare(b, false)
            .declare(c, false)
            .declare(d, false);
        var s1 = s0
            .write(null, a, Type('int'), new SsaNode<Var, Type>(null), h)
            .write(null, b, Type('int'), new SsaNode<Var, Type>(null), h);
        var s2 = s0
            .write(null, a, Type('int'), new SsaNode<Var, Type>(null), h)
            .write(null, c, Type('int'), new SsaNode<Var, Type>(null), h);
        var result = s1.rebaseForward(h, s2);
        expect(result.infoFor(a).assigned, true);
        expect(result.infoFor(b).assigned, true);
        expect(result.infoFor(c).assigned, true);
        expect(result.infoFor(d).assigned, false);
      });

      test('write captured', () {
        var h = Harness();
        var a = Var('a', 'int');
        var b = Var('b', 'int');
        var c = Var('c', 'int');
        var d = Var('d', 'int');
        var s0 = FlowModel<Var, Type>(Reachability.initial)
            .declare(a, false)
            .declare(b, false)
            .declare(c, false)
            .declare(d, false);
        // In s1, a and b are write captured.  In s2, a and c are.
        var s1 = s0.conservativeJoin([a, b], [a, b]);
        var s2 = s1.conservativeJoin([a, c], [a, c]);
        var result = s1.rebaseForward(h, s2);
        expect(
          result.infoFor(a),
          _matchVariableModel(writeCaptured: true, unassigned: false),
        );
        expect(
          result.infoFor(b),
          _matchVariableModel(writeCaptured: true, unassigned: false),
        );
        expect(
          result.infoFor(c),
          _matchVariableModel(writeCaptured: true, unassigned: false),
        );
        expect(
          result.infoFor(d),
          _matchVariableModel(writeCaptured: false, unassigned: true),
        );
      });

      test('write captured and promoted', () {
        var h = Harness();
        var a = Var('a', 'num');
        var s0 = FlowModel<Var, Type>(Reachability.initial).declare(a, false);
        // In s1, a is write captured.  In s2 it's promoted.
        var s1 = s0.conservativeJoin([a], [a]);
        var s2 = s0._tryPromoteForTypeCheck(h, a, 'int').ifTrue;
        expect(
          s1.rebaseForward(h, s2).infoFor(a),
          _matchVariableModel(writeCaptured: true, chain: isNull),
        );
        expect(
          s2.rebaseForward(h, s1).infoFor(a),
          _matchVariableModel(writeCaptured: true, chain: isNull),
        );
      });

      test('promotion', () {
        void _check(String? thisType, String? otherType, bool unsafe,
            List<String>? expectedChain) {
          var h = Harness();
          var x = Var('x', 'Object?');
          var s0 = FlowModel<Var, Type>(Reachability.initial).declare(x, true);
          var s1 = s0;
          if (unsafe) {
            s1 = s1.write(
                null, x, Type('Object?'), new SsaNode<Var, Type>(null), h);
          }
          if (thisType != null) {
            s1 = s1._tryPromoteForTypeCheck(h, x, thisType).ifTrue;
          }
          var s2 = otherType == null
              ? s0
              : s0._tryPromoteForTypeCheck(h, x, otherType).ifTrue;
          var result = s2.rebaseForward(h, s1);
          if (expectedChain == null) {
            expect(result.variableInfo, contains(x));
            expect(result.infoFor(x).promotedTypes, isNull);
          } else {
            expect(result.infoFor(x).promotedTypes!.map((t) => t.type).toList(),
                expectedChain);
          }
        }

        _check(null, null, false, null);
        _check(null, null, true, null);
        _check('int', null, false, ['int']);
        _check('int', null, true, ['int']);
        _check(null, 'int', false, ['int']);
        _check(null, 'int', true, null);
        _check('int?', 'int', false, ['int?', 'int']);
        _check('int', 'int?', false, ['int']);
        _check('int', 'String', false, ['int']);
        _check('int?', 'int', true, ['int?']);
        _check('int', 'int?', true, ['int']);
        _check('int', 'String', true, ['int']);
      });

      test('promotion chains', () {
        // Verify that the given promotion chain matches the expected list of
        // strings.
        void _checkChain(List<Type>? chain, List<String> expected) {
          var strings = (chain ?? <Type>[]).map((t) => t.type).toList();
          expect(strings, expected);
        }

        // Test the following scenario:
        // - Prior to the try/finally block, the sequence of promotions in
        //   [before] is done.
        // - During the try block, the sequence of promotions in [inTry] is
        //   done.
        // - During the finally block, the sequence of promotions in
        //   [inFinally] is done.
        // - After calling `restrict` to refine the state from the finally
        //   block, the expected promotion chain is [expectedResult].
        void _check(List<String> before, List<String> inTry,
            List<String> inFinally, List<String> expectedResult) {
          var h = Harness();
          var x = Var('x', 'Object?');
          var initialModel =
              FlowModel<Var, Type>(Reachability.initial).declare(x, true);
          for (var t in before) {
            initialModel = initialModel._tryPromoteForTypeCheck(h, x, t).ifTrue;
          }
          _checkChain(initialModel.infoFor(x).promotedTypes, before);
          var tryModel = initialModel;
          for (var t in inTry) {
            tryModel = tryModel._tryPromoteForTypeCheck(h, x, t).ifTrue;
          }
          var expectedTryChain = before.toList()..addAll(inTry);
          _checkChain(tryModel.infoFor(x).promotedTypes, expectedTryChain);
          var finallyModel = initialModel;
          for (var t in inFinally) {
            finallyModel = finallyModel._tryPromoteForTypeCheck(h, x, t).ifTrue;
          }
          var expectedFinallyChain = before.toList()..addAll(inFinally);
          _checkChain(
              finallyModel.infoFor(x).promotedTypes, expectedFinallyChain);
          var result = tryModel.rebaseForward(h, finallyModel);
          _checkChain(result.infoFor(x).promotedTypes, expectedResult);
          // And verify that the inputs are unchanged.
          _checkChain(initialModel.infoFor(x).promotedTypes, before);
          _checkChain(tryModel.infoFor(x).promotedTypes, expectedTryChain);
          _checkChain(
              finallyModel.infoFor(x).promotedTypes, expectedFinallyChain);
        }

        _check(['Object'], ['num', 'int'], ['Iterable', 'List'],
            ['Object', 'Iterable', 'List']);
        _check([], ['num', 'int'], ['Iterable', 'List'], ['Iterable', 'List']);
        _check(['Object'], [], ['Iterable', 'List'],
            ['Object', 'Iterable', 'List']);
        _check([], [], ['Iterable', 'List'], ['Iterable', 'List']);
        _check(['Object'], ['num', 'int'], [], ['Object', 'num', 'int']);
        _check([], ['num', 'int'], [], ['num', 'int']);
        _check(['Object'], [], [], ['Object']);
        _check([], [], [], []);
        _check(
            [], ['num', 'int'], ['Object', 'Iterable'], ['Object', 'Iterable']);
        _check([], ['num', 'int'], ['Object'], ['Object', 'num', 'int']);
        _check([], ['Object', 'Iterable'], ['num', 'int'], ['num', 'int']);
        _check([], ['Object'], ['num', 'int'], ['num', 'int']);
        _check([], ['num'], ['Object', 'int'], ['Object', 'int']);
        _check([], ['int'], ['Object', 'num'], ['Object', 'num', 'int']);
        _check([], ['Object', 'int'], ['num'], ['num', 'int']);
        _check([], ['Object', 'num'], ['int'], ['int']);
      });

      test('types of interest', () {
        var h = Harness();
        var a = Var('a', 'Object');
        var s0 = FlowModel<Var, Type>(Reachability.initial).declare(a, false);
        var s1 = s0._tryPromoteForTypeCheck(h, a, 'int').ifFalse;
        var s2 = s0._tryPromoteForTypeCheck(h, a, 'String').ifFalse;
        expect(
          s1.rebaseForward(h, s2).infoFor(a),
          _matchVariableModel(ofInterest: ['int', 'String']),
        );
        expect(
          s2.rebaseForward(h, s1).infoFor(a),
          _matchVariableModel(ofInterest: ['int', 'String']),
        );
      });

      test('variable present in one state but not the other', () {
        var h = Harness();
        var x = Var('x', 'Object?');
        var s0 = FlowModel<Var, Type>(Reachability.initial);
        var s1 = s0.declare(x, true);
        expect(s1.rebaseForward(h, s0), same(s0));
        expect(s0.rebaseForward(h, s1), same(s1));
      });
    });
  });

  group('joinPromotionChains', () {
    var doubleType = Type('double');
    var intType = Type('int');
    var numType = Type('num');
    var objectType = Type('Object');

    test('should handle nulls', () {
      var h = Harness();
      expect(VariableModel.joinPromotedTypes(null, null, h), null);
      expect(VariableModel.joinPromotedTypes(null, [intType], h), null);
      expect(VariableModel.joinPromotedTypes([intType], null, h), null);
    });

    test('should return null if there are no common types', () {
      var h = Harness();
      expect(VariableModel.joinPromotedTypes([intType], [doubleType], h), null);
    });

    test('should return common prefix if there are common types', () {
      var h = Harness();
      expect(
          VariableModel.joinPromotedTypes(
              [objectType, intType], [objectType, doubleType], h),
          _matchPromotionChain(['Object']));
      expect(
          VariableModel.joinPromotedTypes([objectType, numType, intType],
              [objectType, numType, doubleType], h),
          _matchPromotionChain(['Object', 'num']));
    });

    test('should return an input if it is a prefix of the other', () {
      var h = Harness();
      var prefix = [objectType, numType];
      var largerChain = [objectType, numType, intType];
      expect(VariableModel.joinPromotedTypes(prefix, largerChain, h),
          same(prefix));
      expect(VariableModel.joinPromotedTypes(largerChain, prefix, h),
          same(prefix));
      expect(VariableModel.joinPromotedTypes(prefix, prefix, h), same(prefix));
    });

    test('should intersect', () {
      var h = Harness();

      // F <: E <: D <: C <: B <: A
      var A = Type('A');
      var B = Type('B');
      var C = Type('C');
      var D = Type('D');
      var E = Type('E');
      var F = Type('F');
      h.addSubtype('A', 'B', false);
      h.addSubtype('B', 'A', true);
      h.addSubtype('B', 'C', false);
      h.addSubtype('B', 'D', false);
      h.addSubtype('C', 'B', true);
      h.addSubtype('C', 'D', false);
      h.addSubtype('C', 'E', false);
      h.addSubtype('D', 'B', true);
      h.addSubtype('D', 'C', true);
      h.addSubtype('D', 'E', false);
      h.addSubtype('D', 'F', false);
      h.addSubtype('E', 'C', true);
      h.addSubtype('E', 'D', true);
      h.addSubtype('E', 'F', false);
      h.addSubtype('F', 'D', true);
      h.addSubtype('F', 'E', true);

      void check(List<Type> chain1, List<Type> chain2, Matcher matcher) {
        expect(
          VariableModel.joinPromotedTypes(chain1, chain2, h),
          matcher,
        );

        expect(
          VariableModel.joinPromotedTypes(chain2, chain1, h),
          matcher,
        );
      }

      {
        var chain1 = [A, B, C];
        var chain2 = [A, C];
        check(chain1, chain2, same(chain2));
      }

      check(
        [A, B, C, F],
        [A, D, E, F],
        _matchPromotionChain(['A', 'F']),
      );

      check(
        [A, B, E, F],
        [A, C, D, F],
        _matchPromotionChain(['A', 'F']),
      );

      check(
        [A, C, E],
        [B, C, D],
        _matchPromotionChain(['C']),
      );

      check(
        [A, C, E, F],
        [B, C, D, F],
        _matchPromotionChain(['C', 'F']),
      );

      check(
        [A, B, C],
        [A, B, D],
        _matchPromotionChain(['A', 'B']),
      );
    });
  });

  group('joinTypesOfInterest', () {
    List<Type> _makeTypes(List<String> typeNames) =>
        typeNames.map((t) => Type(t)).toList();

    test('simple prefix', () {
      var h = Harness();
      var s1 = _makeTypes(['double', 'int']);
      var s2 = _makeTypes(['double', 'int', 'bool']);
      var expected = _matchOfInterestSet(['double', 'int', 'bool']);
      expect(VariableModel.joinTested(s1, s2, h), expected);
      expect(VariableModel.joinTested(s2, s1, h), expected);
    });

    test('common prefix', () {
      var h = Harness();
      var s1 = _makeTypes(['double', 'int', 'String']);
      var s2 = _makeTypes(['double', 'int', 'bool']);
      var expected = _matchOfInterestSet(['double', 'int', 'String', 'bool']);
      expect(VariableModel.joinTested(s1, s2, h), expected);
      expect(VariableModel.joinTested(s2, s1, h), expected);
    });

    test('order mismatch', () {
      var h = Harness();
      var s1 = _makeTypes(['double', 'int']);
      var s2 = _makeTypes(['int', 'double']);
      var expected = _matchOfInterestSet(['double', 'int']);
      expect(VariableModel.joinTested(s1, s2, h), expected);
      expect(VariableModel.joinTested(s2, s1, h), expected);
    });

    test('small common prefix', () {
      var h = Harness();
      var s1 = _makeTypes(['int', 'double', 'String', 'bool']);
      var s2 = _makeTypes(['int', 'List', 'bool', 'Future']);
      var expected = _matchOfInterestSet(
          ['int', 'double', 'String', 'bool', 'List', 'Future']);
      expect(VariableModel.joinTested(s1, s2, h), expected);
      expect(VariableModel.joinTested(s2, s1, h), expected);
    });
  });

  group('join', () {
    var x = Var('x', 'Object?');
    var y = Var('y', 'Object?');
    var z = Var('z', 'Object?');
    var w = Var('w', 'Object?');
    var intType = Type('int');
    var intQType = Type('int?');
    var stringType = Type('String');
    const emptyMap = const <Var, VariableModel<Var, Type>>{};

    VariableModel<Var, Type> model(List<Type>? promotionChain,
            {List<Type>? typesOfInterest, bool assigned = false}) =>
        VariableModel<Var, Type>(
            promotedTypes: promotionChain,
            tested: typesOfInterest ?? promotionChain ?? [],
            assigned: assigned,
            unassigned: !assigned,
            ssaNode: new SsaNode<Var, Type>(null));

    group('without input reuse', () {
      test('promoted with unpromoted', () {
        var h = Harness();
        var p1 = {
          x: model([intType]),
          y: model(null)
        };
        var p2 = {
          x: model(null),
          y: model([intType])
        };
        expect(FlowModel.joinVariableInfo(h, p1, p2, emptyMap), {
          x: _matchVariableModel(chain: null, ofInterest: ['int']),
          y: _matchVariableModel(chain: null, ofInterest: ['int'])
        });
      });
    });
    group('should re-use an input if possible', () {
      test('identical inputs', () {
        var h = Harness();
        var p = {
          x: model([intType]),
          y: model([stringType])
        };
        expect(FlowModel.joinVariableInfo(h, p, p, emptyMap), same(p));
      });

      test('one input empty', () {
        var h = Harness();
        var p1 = {
          x: model([intType]),
          y: model([stringType])
        };
        var p2 = <Var, VariableModel<Var, Type>>{};
        expect(FlowModel.joinVariableInfo(h, p1, p2, emptyMap), same(emptyMap));
        expect(FlowModel.joinVariableInfo(h, p2, p1, emptyMap), same(emptyMap));
      });

      test('promoted with unpromoted', () {
        var h = Harness();
        var p1 = {
          x: model([intType])
        };
        var p2 = {x: model(null)};
        var expected = {
          x: _matchVariableModel(chain: null, ofInterest: ['int'])
        };
        expect(FlowModel.joinVariableInfo(h, p1, p2, emptyMap), expected);
        expect(FlowModel.joinVariableInfo(h, p2, p1, emptyMap), expected);
      });

      test('related type chains', () {
        var h = Harness();
        var p1 = {
          x: model([intQType, intType])
        };
        var p2 = {
          x: model([intQType])
        };
        var expected = {
          x: _matchVariableModel(chain: ['int?'], ofInterest: ['int?', 'int'])
        };
        expect(FlowModel.joinVariableInfo(h, p1, p2, emptyMap), expected);
        expect(FlowModel.joinVariableInfo(h, p2, p1, emptyMap), expected);
      });

      test('unrelated type chains', () {
        var h = Harness();
        var p1 = {
          x: model([intType])
        };
        var p2 = {
          x: model([stringType])
        };
        var expected = {
          x: _matchVariableModel(chain: null, ofInterest: ['String', 'int'])
        };
        expect(FlowModel.joinVariableInfo(h, p1, p2, emptyMap), expected);
        expect(FlowModel.joinVariableInfo(h, p2, p1, emptyMap), expected);
      });

      test('sub-map', () {
        var h = Harness();
        var xModel = model([intType]);
        var p1 = {
          x: xModel,
          y: model([stringType])
        };
        var p2 = {x: xModel};
        expect(FlowModel.joinVariableInfo(h, p1, p2, emptyMap), same(p2));
        expect(FlowModel.joinVariableInfo(h, p2, p1, emptyMap), same(p2));
      });

      test('sub-map with matched subtype', () {
        var h = Harness();
        var p1 = {
          x: model([intQType, intType]),
          y: model([stringType])
        };
        var p2 = {
          x: model([intQType])
        };
        var expected = {
          x: _matchVariableModel(chain: ['int?'], ofInterest: ['int?', 'int'])
        };
        expect(FlowModel.joinVariableInfo(h, p1, p2, emptyMap), expected);
        expect(FlowModel.joinVariableInfo(h, p2, p1, emptyMap), expected);
      });

      test('sub-map with mismatched subtype', () {
        var h = Harness();
        var p1 = {
          x: model([intQType]),
          y: model([stringType])
        };
        var p2 = {
          x: model([intQType, intType])
        };
        var expected = {
          x: _matchVariableModel(chain: ['int?'], ofInterest: ['int?', 'int'])
        };
        expect(FlowModel.joinVariableInfo(h, p1, p2, emptyMap), expected);
        expect(FlowModel.joinVariableInfo(h, p2, p1, emptyMap), expected);
      });

      test('assigned', () {
        var h = Harness();
        var unassigned = model(null, assigned: false);
        var assigned = model(null, assigned: true);
        var p1 = {x: assigned, y: assigned, z: unassigned, w: unassigned};
        var p2 = {x: assigned, y: unassigned, z: assigned, w: unassigned};
        var joined = FlowModel.joinVariableInfo(h, p1, p2, emptyMap);
        expect(joined, {
          x: same(assigned),
          y: _matchVariableModel(
              chain: null, assigned: false, unassigned: false),
          z: _matchVariableModel(
              chain: null, assigned: false, unassigned: false),
          w: same(unassigned)
        });
      });

      test('write captured', () {
        var h = Harness();
        var intQModel = model([intQType]);
        var writeCapturedModel = intQModel.writeCapture();
        var p1 = {
          x: writeCapturedModel,
          y: writeCapturedModel,
          z: intQModel,
          w: intQModel
        };
        var p2 = {
          x: writeCapturedModel,
          y: intQModel,
          z: writeCapturedModel,
          w: intQModel
        };
        var joined = FlowModel.joinVariableInfo(h, p1, p2, emptyMap);
        expect(joined, {
          x: same(writeCapturedModel),
          y: same(writeCapturedModel),
          z: same(writeCapturedModel),
          w: same(intQModel)
        });
      });
    });
  });

  group('merge', () {
    var x = Var('x', 'Object?');
    var intType = Type('int');
    var stringType = Type('String');
    const emptyMap = const <Var, VariableModel<Var, Type>>{};

    VariableModel<Var, Type> varModel(List<Type>? promotionChain,
            {bool assigned = false}) =>
        VariableModel<Var, Type>(
            promotedTypes: promotionChain,
            tested: promotionChain ?? [],
            assigned: assigned,
            unassigned: !assigned,
            ssaNode: new SsaNode<Var, Type>(null));

    test('first is null', () {
      var h = Harness();
      var s1 = FlowModel.withInfo(Reachability.initial.split(), emptyMap);
      var result = FlowModel.merge(h, null, s1, emptyMap);
      expect(result.reachable, same(Reachability.initial));
    });

    test('second is null', () {
      var h = Harness();
      var splitPoint = Reachability.initial.split();
      var afterSplit = splitPoint.split();
      var s1 = FlowModel.withInfo(afterSplit, emptyMap);
      var result = FlowModel.merge(h, s1, null, emptyMap);
      expect(result.reachable, same(splitPoint));
    });

    test('both are reachable', () {
      var h = Harness();
      var splitPoint = Reachability.initial.split();
      var afterSplit = splitPoint.split();
      var s1 = FlowModel.withInfo(afterSplit, {
        x: varModel([intType])
      });
      var s2 = FlowModel.withInfo(afterSplit, {
        x: varModel([stringType])
      });
      var result = FlowModel.merge(h, s1, s2, emptyMap);
      expect(result.reachable, same(splitPoint));
      expect(result.variableInfo[x]!.promotedTypes, isNull);
    });

    test('first is unreachable', () {
      var h = Harness();
      var splitPoint = Reachability.initial.split();
      var afterSplit = splitPoint.split();
      var s1 = FlowModel.withInfo(afterSplit.setUnreachable(), {
        x: varModel([intType])
      });
      var s2 = FlowModel.withInfo(afterSplit, {
        x: varModel([stringType])
      });
      var result = FlowModel.merge(h, s1, s2, emptyMap);
      expect(result.reachable, same(splitPoint));
      expect(result.variableInfo, same(s2.variableInfo));
    });

    test('second is unreachable', () {
      var h = Harness();
      var splitPoint = Reachability.initial.split();
      var afterSplit = splitPoint.split();
      var s1 = FlowModel.withInfo(afterSplit, {
        x: varModel([intType])
      });
      var s2 = FlowModel.withInfo(afterSplit.setUnreachable(), {
        x: varModel([stringType])
      });
      var result = FlowModel.merge(h, s1, s2, emptyMap);
      expect(result.reachable, same(splitPoint));
      expect(result.variableInfo, same(s1.variableInfo));
    });

    test('both are unreachable', () {
      var h = Harness();
      var splitPoint = Reachability.initial.split();
      var afterSplit = splitPoint.split();
      var s1 = FlowModel.withInfo(afterSplit.setUnreachable(), {
        x: varModel([intType])
      });
      var s2 = FlowModel.withInfo(afterSplit.setUnreachable(), {
        x: varModel([stringType])
      });
      var result = FlowModel.merge(h, s1, s2, emptyMap);
      expect(result.reachable.locallyReachable, false);
      expect(result.reachable.parent, same(splitPoint.parent));
      expect(result.variableInfo[x]!.promotedTypes, isNull);
    });
  });

  group('inheritTested', () {
    var x = Var('x', 'Object?');
    var intType = Type('int');
    var stringType = Type('String');
    const emptyMap = const <Var, VariableModel<Var, Type>>{};

    VariableModel<Var, Type> model(List<Type> typesOfInterest) =>
        VariableModel<Var, Type>(
            promotedTypes: null,
            tested: typesOfInterest,
            assigned: true,
            unassigned: false,
            ssaNode: new SsaNode<Var, Type>(null));

    test('inherits types of interest from other', () {
      var h = Harness();
      var m1 = FlowModel.withInfo(Reachability.initial, {
        x: model([intType])
      });
      var m2 = FlowModel.withInfo(Reachability.initial, {
        x: model([stringType])
      });
      expect(m1.inheritTested(h, m2).variableInfo[x]!.tested,
          _matchOfInterestSet(['int', 'String']));
    });

    test('handles variable missing from other', () {
      var h = Harness();
      var m1 = FlowModel.withInfo(Reachability.initial, {
        x: model([intType])
      });
      var m2 = FlowModel.withInfo(Reachability.initial, emptyMap);
      expect(m1.inheritTested(h, m2), same(m1));
    });

    test('returns identical model when no changes', () {
      var h = Harness();
      var m1 = FlowModel.withInfo(Reachability.initial, {
        x: model([intType])
      });
      var m2 = FlowModel.withInfo(Reachability.initial, {
        x: model([intType])
      });
      expect(m1.inheritTested(h, m2), same(m1));
    });
  });

  group('Legacy promotion', () {
    group('if statement', () {
      group('promotes a variable whose type is shown by its condition', () {
        test('within then-block', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(x.read.is_('int'), [
              checkPromoted(x, 'int'),
            ]),
          ]);
        });

        test('but not within else-block', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(x.read.is_('int'), [], [
              checkNotPromoted(x),
            ]),
          ]);
        });

        test('unless the then-block mutates it', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(x.read.is_('int'), [
              checkNotPromoted(x),
              x.write(expr('int')).stmt,
            ]),
          ]);
        });

        test('even if the condition mutates it', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(
                x
                    .write(expr('int'))
                    .parenthesized
                    .eq(expr('int'))
                    .and(x.read.is_('int')),
                [
                  checkPromoted(x, 'int'),
                ]),
          ]);
        });

        test('even if the else-block mutates it', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(x.read.is_('int'), [
              checkPromoted(x, 'int'),
            ], [
              x.write(expr('int')).stmt,
            ]),
          ]);
        });

        test('unless a closure mutates it', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(x.read.is_('int'), [
              checkNotPromoted(x),
            ]),
            localFunction([
              x.write(expr('int')).stmt,
            ]),
          ]);
        });

        test(
            'unless a closure in the then-block accesses it and it is mutated '
            'anywhere', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(x.read.is_('int'), [
              checkNotPromoted(x),
              localFunction([
                x.read.stmt,
              ]),
            ]),
            x.write(expr('int')).stmt,
          ]);
        });

        test(
            'unless a closure in the then-block accesses it and it is mutated '
            'anywhere, even if the access is deeply nested', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(x.read.is_('int'), [
              checkNotPromoted(x),
              localFunction([
                localFunction([
                  x.read.stmt,
                ]),
              ]),
            ]),
            x.write(expr('int')).stmt,
          ]);
        });

        test(
            'even if a closure in the condition accesses it and it is mutated '
            'somewhere', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(
                localFunction([
                  x.read.stmt,
                ]).thenExpr(expr('bool')).and(x.read.is_('int')),
                [
                  checkPromoted(x, 'int'),
                ]),
            x.write(expr('int')).stmt,
          ]);
        });

        test(
            'even if a closure in the else-block accesses it and it is mutated '
            'somewhere', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(x.read.is_('int'), [
              checkPromoted(x, 'int'),
            ], [
              localFunction([
                x.read.stmt,
              ]),
            ]),
            x.write(expr('int')).stmt,
          ]);
        });

        test(
            'even if a closure in the then-block accesses it, provided it is '
            'not mutated anywhere', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(x.read.is_('int'), [
              checkPromoted(x, 'int'),
              localFunction([
                x.read.stmt,
              ]),
            ]),
          ]);
        });
      });

      test('handles arbitrary conditions', () {
        var h = Harness(legacy: true);
        h.run([
          if_(expr('bool'), []),
        ]);
      });

      test('handles a condition that is a variable', () {
        var h = Harness(legacy: true);
        var x = Var('x', 'bool');
        h.run([
          if_(x.read, []),
        ]);
      });

      test('handles multiple promotions', () {
        var h = Harness(legacy: true);
        var x = Var('x', 'Object');
        var y = Var('y', 'Object');
        h.run([
          if_(x.read.is_('int').and(y.read.is_('String')), [
            checkPromoted(x, 'int'),
            checkPromoted(y, 'String'),
          ]),
        ]);
      });
    });

    group('conditional expression', () {
      group('promotes a variable whose type is shown by its condition', () {
        test('within then-expression', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            x.read
                .is_('int')
                .conditional(checkPromoted(x, 'int').thenExpr(expr('Object')),
                    expr('Object'))
                .stmt,
          ]);
        });

        test('but not within else-expression', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            x.read
                .is_('int')
                .conditional(expr('Object'),
                    checkNotPromoted(x).thenExpr(expr('Object')))
                .stmt,
          ]);
        });

        test('unless the then-expression mutates it', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            x.read
                .is_('int')
                .conditional(
                    block([
                      checkNotPromoted(x),
                      x.write(expr('int')).stmt,
                    ]).thenExpr(expr('Object')),
                    expr('Object'))
                .stmt,
          ]);
        });

        test('even if the condition mutates it', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            x
                .write(expr('int'))
                .parenthesized
                .eq(expr('int'))
                .and(x.read.is_('int'))
                .conditional(checkPromoted(x, 'int').thenExpr(expr('Object')),
                    expr('Object'))
                .stmt,
          ]);
        });

        test('even if the else-expression mutates it', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            x.read
                .is_('int')
                .conditional(checkPromoted(x, 'int').thenExpr(expr('int')),
                    x.write(expr('int')))
                .stmt,
          ]);
        });

        test('unless a closure mutates it', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            x.read
                .is_('int')
                .conditional(checkNotPromoted(x).thenExpr(expr('Object')),
                    expr('Object'))
                .stmt,
            localFunction([
              x.write(expr('int')).stmt,
            ]),
          ]);
        });

        test(
            'unless a closure in the then-expression accesses it and it is '
            'mutated anywhere', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            x.read
                .is_('int')
                .conditional(
                    block([
                      checkNotPromoted(x),
                      localFunction([
                        x.read.stmt,
                      ]),
                    ]).thenExpr(expr('Object')),
                    expr('Object'))
                .stmt,
            x.write(expr('int')).stmt,
          ]);
        });

        test(
            'even if a closure in the condition accesses it and it is mutated '
            'somewhere', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            localFunction([
              x.read.stmt,
            ])
                .thenExpr(expr('Object'))
                .and(x.read.is_('int'))
                .conditional(checkPromoted(x, 'int').thenExpr(expr('Object')),
                    expr('Object'))
                .stmt,
            x.write(expr('int')).stmt,
          ]);
        });

        test(
            'even if a closure in the else-expression accesses it and it is '
            'mutated somewhere', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            x.read
                .is_('int')
                .conditional(
                    checkPromoted(x, 'int').thenExpr(expr('Object')),
                    localFunction([
                      x.read.stmt,
                    ]).thenExpr(expr('Object')))
                .stmt,
            x.write(expr('int')).stmt,
          ]);
        });

        test(
            'even if a closure in the then-expression accesses it, provided it '
            'is not mutated anywhere', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            x.read
                .is_('int')
                .conditional(
                    block([
                      checkPromoted(x, 'int'),
                      localFunction([
                        x.read.stmt,
                      ]),
                    ]).thenExpr(expr('Object')),
                    expr('Object'))
                .stmt,
          ]);
        });
      });

      test('handles arbitrary conditions', () {
        var h = Harness(legacy: true);
        h.run([
          expr('bool').conditional(expr('Object'), expr('Object')).stmt,
        ]);
      });

      test('handles a condition that is a variable', () {
        var h = Harness(legacy: true);
        var x = Var('x', 'bool');
        h.run([
          x.read.conditional(expr('Object'), expr('Object')).stmt,
        ]);
      });

      test('handles multiple promotions', () {
        var h = Harness(legacy: true);
        var x = Var('x', 'Object');
        var y = Var('y', 'Object');
        h.run([
          x.read
              .is_('int')
              .and(y.read.is_('String'))
              .conditional(
                  block([
                    checkPromoted(x, 'int'),
                    checkPromoted(y, 'String'),
                  ]).thenExpr(expr('Object')),
                  expr('Object'))
              .stmt
        ]);
      });
    });

    group('logical', () {
      group('and', () {
        group("shows a variable's type", () {
          test('if the lhs shows the type', () {
            var h = Harness(legacy: true);
            var x = Var('x', 'Object');
            h.run([
              if_(x.read.is_('int').and(expr('bool')), [
                checkPromoted(x, 'int'),
              ]),
            ]);
          });

          test('if the rhs shows the type', () {
            var h = Harness(legacy: true);
            var x = Var('x', 'Object');
            h.run([
              if_(expr('bool').and(x.read.is_('int')), [
                checkPromoted(x, 'int'),
              ]),
            ]);
          });

          test('unless the rhs mutates it', () {
            var h = Harness(legacy: true);
            var x = Var('x', 'Object');
            h.run([
              if_(x.read.is_('int').and(x.write(expr('bool'))), [
                checkNotPromoted(x),
              ]),
            ]);
          });

          test('unless the rhs mutates it, even if the rhs also shows the type',
              () {
            var h = Harness(legacy: true);
            var x = Var('x', 'Object');
            h.run([
              if_(
                  expr('bool').and(x
                      .write(expr('Object'))
                      .and(x.read.is_('int'))
                      .parenthesized),
                  [
                    checkNotPromoted(x),
                  ]),
            ]);
          });

          test('unless a closure mutates it', () {
            var h = Harness(legacy: true);
            var x = Var('x', 'Object');
            h.run([
              if_(x.read.is_('int').and(expr('bool')), [
                checkNotPromoted(x),
              ]),
              localFunction([
                x.write(expr('int')).stmt,
              ]),
            ]);
          });
        });

        group('promotes a variable whose type is shown by its lhs', () {
          test('within its rhs', () {
            var h = Harness(legacy: true);
            var x = Var('x', 'Object');
            h.run([
              x.read
                  .is_('int')
                  .and(checkPromoted(x, 'int').thenExpr(expr('bool')))
                  .stmt,
            ]);
          });

          test('unless the lhs mutates it', () {
            var h = Harness(legacy: true);
            var x = Var('x', 'Object');
            h.run([
              x
                  .write(expr('int'))
                  .parenthesized
                  .eq(expr('int'))
                  .and(x.read.is_('int'))
                  .parenthesized
                  .and(checkNotPromoted(x).thenExpr(expr('bool')))
                  .stmt,
            ]);
          });

          test('unless the rhs mutates it', () {
            var h = Harness(legacy: true);
            var x = Var('x', 'Object');
            h.run([
              x.read
                  .is_('int')
                  .and(checkNotPromoted(x).thenExpr(x.write(expr('bool'))))
                  .stmt,
            ]);
          });

          test('unless a closure mutates it', () {
            var h = Harness(legacy: true);
            var x = Var('x', 'Object');
            h.run([
              x.read
                  .is_('int')
                  .and(checkNotPromoted(x).thenExpr(expr('bool')))
                  .stmt,
              localFunction([
                x.write(expr('int')).stmt,
              ]),
            ]);
          });

          test(
              'unless a closure in the rhs accesses it and it is mutated '
              'anywhere', () {
            var h = Harness(legacy: true);
            var x = Var('x', 'Object');
            h.run([
              x.read
                  .is_('int')
                  .and(block([
                    checkNotPromoted(x),
                    localFunction([
                      x.read.stmt,
                    ]),
                  ]).thenExpr(expr('bool')))
                  .stmt,
              x.write(expr('int')).stmt,
            ]);
          });

          test(
              'even if a closure in the lhs accesses it and it is mutated '
              'somewhere', () {
            var h = Harness(legacy: true);
            var x = Var('x', 'Object');
            h.run([
              localFunction([
                x.read.stmt,
              ])
                  .thenExpr(expr('Object'))
                  .and(x.read.is_('int'))
                  .parenthesized
                  .and(checkPromoted(x, 'int').thenExpr(expr('bool')))
                  .stmt,
              x.write(expr('int')).stmt,
            ]);
          });

          test(
              'even if a closure in the rhs accesses it, provided it is not '
              'mutated anywhere', () {
            var h = Harness(legacy: true);
            var x = Var('x', 'Object');
            h.run([
              x.read
                  .is_('int')
                  .and(block([
                    checkPromoted(x, 'int'),
                    localFunction([
                      x.read.stmt,
                    ]),
                  ]).thenExpr(expr('bool')))
                  .stmt,
            ]);
          });
        });

        test('uses lhs promotion if rhs is not to a subtype', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          // Note: for this to be an effective test, we need to mutate `x` on
          // the LHS of the outer `&&` so that `x` is not promoted on the RHS
          // (and thus the lesser promotion on the RHS can take effect).
          h.run([
            if_(
                x
                    .write(expr('Object'))
                    .parenthesized
                    .and(x.read.is_('int'))
                    .parenthesized
                    .and(x.read.is_('num')),
                [
                  checkPromoted(x, 'int'),
                ]),
          ]);
        });

        test('uses rhs promotion if rhs is to a subtype', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(x.read.is_('num').and(x.read.is_('int')), [
              checkPromoted(x, 'int'),
            ]),
          ]);
        });

        test('can handle multiple promotions on lhs', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          var y = Var('y', 'Object');
          h.run([
            x.read
                .is_('int')
                .and(y.read.is_('String'))
                .parenthesized
                .and(block([
                  checkPromoted(x, 'int'),
                  checkPromoted(y, 'String'),
                ]).thenExpr(expr('bool')))
                .stmt,
          ]);
        });

        test('handles variables', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'bool');
          var y = Var('y', 'bool');
          h.run([
            if_(x.read.and(y.read), []),
          ]);
        });

        test('handles arbitrary expressions', () {
          var h = Harness(legacy: true);
          h.run([
            if_(expr('bool').and(expr('bool')), []),
          ]);
        });
      });

      test('or is ignored', () {
        var h = Harness(legacy: true);
        var x = Var('x', 'Object');
        h.run([
          if_(x.read.is_('int').or(x.read.is_('int')), [
            checkNotPromoted(x),
          ], [
            checkNotPromoted(x),
          ])
        ]);
      });
    });

    group('is test', () {
      group("shows a variable's type", () {
        test('normally', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(x.read.is_('int'), [
              checkPromoted(x, 'int'),
            ], [
              checkNotPromoted(x),
            ])
          ]);
        });

        test('unless the test is inverted', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(x.read.is_('int', isInverted: true), [
              checkNotPromoted(x),
            ], [
              checkNotPromoted(x),
            ])
          ]);
        });

        test('unless the tested type is not a subtype of the declared type',
            () {
          var h = Harness(legacy: true);
          var x = Var('x', 'String');
          h.run([
            if_(x.read.is_('int'), [
              checkNotPromoted(x),
            ], [
              checkNotPromoted(x),
            ])
          ]);
        });

        test("even when the variable's type has been previously promoted", () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(x.read.is_('num'), [
              if_(x.read.is_('int'), [
                checkPromoted(x, 'int'),
              ], [
                checkPromoted(x, 'num'),
              ])
            ]),
          ]);
        });

        test(
            'unless the tested type is not a subtype of the previously '
            'promoted type', () {
          var h = Harness(legacy: true);
          var x = Var('x', 'Object');
          h.run([
            if_(x.read.is_('String'), [
              if_(x.read.is_('int'), [
                checkPromoted(x, 'String'),
              ], [
                checkPromoted(x, 'String'),
              ])
            ]),
          ]);
        });

        test('even when the declared type is a type variable', () {
          var h = Harness(legacy: true);
          h.addPromotionException('T', 'int', 'T&int');
          var x = Var('x', 'T');
          h.run([
            if_(x.read.is_('int'), [
              checkPromoted(x, 'T&int'),
            ]),
          ]);
        });
      });

      test('handles arbitrary expressions', () {
        var h = Harness(legacy: true);
        h.run([
          if_(expr('Object').is_('int'), []),
        ]);
      });
    });

    test('forwardExpression does not re-activate a deeply nested expression',
        () {
      var h = Harness(legacy: true);
      var x = Var('x', 'Object');
      h.run([
        if_(x.read.is_('int').eq(expr('Object')).thenStmt(block([])), [
          checkNotPromoted(x),
        ]),
      ]);
    });

    test(
        'parenthesizedExpression does not re-activate a deeply nested '
        'expression', () {
      var h = Harness(legacy: true);
      var x = Var('x', 'Object');
      h.run([
        if_(x.read.is_('int').eq(expr('Object')).parenthesized, [
          checkNotPromoted(x),
        ]),
      ]);
    });

    test('variableRead returns the promoted type if promoted', () {
      var h = Harness(legacy: true);
      var x = Var('x', 'Object');
      h.run([
        if_(
            x
                .readAndCheckPromotedType((type) => expect(type, isNull))
                .is_('int'),
            [
              x
                  .readAndCheckPromotedType((type) => expect(type!.type, 'int'))
                  .stmt,
            ]),
      ]);
    });
  });

  group('why not promoted', () {
    test('due to assignment', () {
      var h = Harness();
      var x = Var('x', 'int?');
      late Expression writeExpression;
      h.run([
        declare(x, initialized: true),
        if_(x.read.eq(nullLiteral), [
          return_(),
        ]),
        checkPromoted(x, 'int'),
        (writeExpression = x.write(expr('int?'))).stmt,
        checkNotPromoted(x),
        x.read.whyNotPromoted((reasons) {
          expect(reasons.keys, unorderedEquals([Type('int')]));
          var nonPromotionReason =
              reasons.values.single as DemoteViaExplicitWrite<Var, Expression>;
          expect(nonPromotionReason.writeExpression, same(writeExpression));
        }).stmt,
      ]);
    });

    test('due to assignment, multiple demotions', () {
      var h = Harness();
      var x = Var('x', 'Object?');
      late Expression writeExpression;
      h.run([
        declare(x, initialized: true),
        if_(x.read.isNot('int?'), [
          return_(),
        ]),
        if_(x.read.eq(nullLiteral), [
          return_(),
        ]),
        checkPromoted(x, 'int'),
        (writeExpression = x.write(expr('Object?'))).stmt,
        checkNotPromoted(x),
        x.read.whyNotPromoted((reasons) {
          expect(reasons.keys, unorderedEquals([Type('int'), Type('int?')]));
          expect(
              (reasons[Type('int')] as DemoteViaExplicitWrite<Var, Expression>)
                  .writeExpression,
              same(writeExpression));
          expect(
              (reasons[Type('int?')] as DemoteViaExplicitWrite<Var, Expression>)
                  .writeExpression,
              same(writeExpression));
        }).stmt,
      ]);
    });

    test('preserved in join when one branch unreachable', () {
      var h = Harness();
      var x = Var('x', 'int?');
      late Expression writeExpression;
      h.run([
        declare(x, initialized: true),
        if_(x.read.eq(nullLiteral), [
          return_(),
        ]),
        checkPromoted(x, 'int'),
        (writeExpression = x.write(expr('int?'))).stmt,
        checkNotPromoted(x),
        if_(expr('bool'), [
          return_(),
        ]),
        x.read.whyNotPromoted((reasons) {
          expect(reasons.keys, unorderedEquals([Type('int')]));
          var nonPromotionReason =
              reasons.values.single as DemoteViaExplicitWrite<Var, Expression>;
          expect(nonPromotionReason.writeExpression, same(writeExpression));
        }).stmt,
      ]);
    });

    test('preserved in later promotions', () {
      var h = Harness();
      var x = Var('x', 'Object');
      late Expression writeExpression;
      h.run([
        declare(x, initialized: true),
        if_(x.read.is_('int', isInverted: true), [
          return_(),
        ]),
        checkPromoted(x, 'int'),
        (writeExpression = x.write(expr('Object'))).stmt,
        checkNotPromoted(x),
        if_(x.read.is_('num', isInverted: true), [
          return_(),
        ]),
        checkPromoted(x, 'num'),
        x.read.whyNotPromoted((reasons) {
          var nonPromotionReason =
              reasons[Type('int')] as DemoteViaExplicitWrite;
          expect(nonPromotionReason.writeExpression, same(writeExpression));
        }).stmt,
      ]);
    });

    test('re-promotion', () {
      var h = Harness();
      var x = Var('x', 'int?');
      h.run([
        declare(x, initialized: true),
        if_(x.read.eq(nullLiteral), [
          return_(),
        ]),
        checkPromoted(x, 'int'),
        x.write(expr('int?')).stmt,
        checkNotPromoted(x),
        if_(x.read.eq(nullLiteral), [
          return_(),
        ]),
        checkPromoted(x, 'int'),
        x.read.whyNotPromoted((reasons) {
          expect(reasons, isEmpty);
        }).stmt,
      ]);
    });

    group('because property', () {
      test('via explicit this', () {
        var h = Harness();
        h.run([
          if_(this_('C').propertyGet('field').eq(nullLiteral), [
            return_(),
          ]),
          this_('C').propertyGet('field').whyNotPromoted((reasons) {
            expect(reasons.keys, unorderedEquals([Type('Object')]));
            var nonPromotionReason = reasons.values.single;
            expect(nonPromotionReason, TypeMatcher<PropertyNotPromoted>());
          }).stmt,
        ]);
      });

      test('via implicit this/super', () {
        var h = Harness();
        h.run([
          if_(thisOrSuperPropertyGet('field').eq(nullLiteral), [
            return_(),
          ]),
          thisOrSuperPropertyGet('field').whyNotPromoted((reasons) {
            expect(reasons.keys, unorderedEquals([Type('Object')]));
            var nonPromotionReason = reasons.values.single;
            expect(nonPromotionReason, TypeMatcher<PropertyNotPromoted>());
          }).stmt,
        ]);
      });

      test('via variable', () {
        var h = Harness();
        var x = Var('x', 'C');
        h.run([
          declare(x, initialized: true),
          if_(x.read.propertyGet('field').eq(nullLiteral), [
            return_(),
          ]),
          x.read.propertyGet('field').whyNotPromoted((reasons) {
            expect(reasons.keys, unorderedEquals([Type('Object')]));
            var nonPromotionReason = reasons.values.single;
            expect(nonPromotionReason, TypeMatcher<PropertyNotPromoted>());
          }).stmt,
        ]);
      });
    });

    group('because this', () {
      test('explicit', () {
        var h = Harness()
          ..addSubtype('D', 'C', true)
          ..addFactor('C', 'D', 'C');
        h.run([
          if_(this_('C').isNot('D'), [
            return_(),
          ]),
          this_('C').whyNotPromoted((reasons) {
            expect(reasons.keys, unorderedEquals([Type('D')]));
            var nonPromotionReason = reasons.values.single;
            expect(nonPromotionReason, TypeMatcher<ThisNotPromoted>());
          }).stmt,
        ]);
      });

      test('implicit', () {
        var h = Harness()
          ..addSubtype('D', 'C', true)
          ..addFactor('C', 'D', 'C');
        h.run([
          if_(this_('C').isNot('D'), [
            return_(),
          ]),
          implicitThis_whyNotPromoted('C', (reasons) {
            expect(reasons.keys, unorderedEquals([Type('D')]));
            var nonPromotionReason = reasons.values.single;
            expect(nonPromotionReason, TypeMatcher<ThisNotPromoted>());
          }),
        ]);
      });
    });
  });
}

/// Returns the appropriate matcher for expecting an assertion error to be
/// thrown or not, based on whether assertions are enabled.
Matcher get _asserts {
  var matcher = throwsA(TypeMatcher<AssertionError>());
  bool assertionsEnabled = false;
  assert(assertionsEnabled = true);
  if (!assertionsEnabled) {
    matcher = isNot(matcher);
  }
  return matcher;
}

String _describeMatcher(Matcher matcher) {
  var description = StringDescription();
  matcher.describe(description);
  return description.toString();
}

Matcher _matchOfInterestSet(List<String> expectedTypes) {
  return predicate(
      (List<Type> x) => unorderedEquals(expectedTypes)
          .matches(x.map((t) => t.type).toList(), {}),
      'interest set $expectedTypes');
}

Matcher _matchPromotionChain(List<String>? expectedTypes) {
  if (expectedTypes == null) return isNull;
  return predicate(
      (List<Type> x) =>
          equals(expectedTypes).matches(x.map((t) => t.type).toList(), {}),
      'promotion chain $expectedTypes');
}

Matcher _matchVariableModel(
    {Object? chain,
    Object? ofInterest,
    Object? assigned,
    Object? unassigned,
    Object? writeCaptured}) {
  chain ??= anything;
  ofInterest ??= anything;
  assigned ??= anything;
  unassigned ??= anything;
  writeCaptured ??= anything;
  Matcher chainMatcher =
      chain is List<String> ? _matchPromotionChain(chain) : wrapMatcher(chain);
  Matcher ofInterestMatcher = ofInterest is List<String>
      ? _matchOfInterestSet(ofInterest)
      : wrapMatcher(ofInterest);
  Matcher assignedMatcher = wrapMatcher(assigned);
  Matcher unassignedMatcher = wrapMatcher(unassigned);
  Matcher writeCapturedMatcher = wrapMatcher(writeCaptured);
  return predicate((VariableModel<Var, Type> model) {
    if (!chainMatcher.matches(model.promotedTypes, {})) return false;
    if (!ofInterestMatcher.matches(model.tested, {})) return false;
    if (!assignedMatcher.matches(model.assigned, {})) return false;
    if (!unassignedMatcher.matches(model.unassigned, {})) return false;
    if (!writeCapturedMatcher.matches(model.writeCaptured, {})) return false;
    return true;
  },
      'VariableModel(chain: ${_describeMatcher(chainMatcher)}, '
      'ofInterest: ${_describeMatcher(ofInterestMatcher)}, '
      'assigned: ${_describeMatcher(assignedMatcher)}, '
      'unassigned: ${_describeMatcher(unassignedMatcher)}, '
      'writeCaptured: ${_describeMatcher(writeCapturedMatcher)})');
}

class _MockNonPromotionReason extends NonPromotionReason {
  String get shortName => fail('Unexpected call to shortName');

  R accept<R, Node extends Object, Expression extends Object,
              Variable extends Object, Type extends Object>(
          NonPromotionReasonVisitor<R, Node, Expression, Variable, Type>
              visitor) =>
      fail('Unexpected call to accept');
}

extension on FlowModel<Var, Type> {
  Reference<Var, Type> _varRef(Var variable) =>
      new VariableReference<Var, Type>(variable);

  ReferenceWithType<Var, Type> _varRefWithType(Var variable) =>
      new ReferenceWithType<Var, Type>(_varRef(variable),
          variableInfo[variable]?.promotedTypes?.last ?? variable.type);

  ExpressionInfo<Var, Type> _tryPromoteForTypeCheck(
          Harness h, Var variable, String type) =>
      tryPromoteForTypeCheck(h, _varRefWithType(variable), Type(type));

  ExpressionInfo<Var, Type> _tryMarkNonNullable(Harness h, Var variable) =>
      tryMarkNonNullable(h, _varRefWithType(variable));
}