tests/language_2/if_null_precedence_test.dart

// Copyright (c) 2015, 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.

// Verify that '??' binds tighter than '?:' and less tightly than '||'.

import "package:expect/expect.dart";

main() {
  // Make sure the "none" test fails if "??" is not implemented.  This makes
  // status files easier to maintain.
  var _ = null ?? null;

  // "a ?? b ?? c" should be legal, and should evaluate to the first non-null
  // value (or null if there are no non-null values).
  Expect.equals(1, 1 ?? 2 ?? 3);
  Expect.equals(2, null ?? 2 ?? 3);
  Expect.equals(3, null ?? null ?? 3);
  Expect.equals(null, null ?? null ?? null);

  // "a ?? b ? c : d" should parse as "(a ?? b) ? c : d", therefore provided
  // that a is true, b need not be a bool.  An incorrect parse of
  // "a ?? (b ? c : d)" would require b to be a bool to avoid a static type
  // warning.
  Expect.equals(2, true ?? 1 ? 2 : 3);

  // "a ?? b || c" should parse as "a ?? (b || c)", therefore it's a static
  // type warning if b doesn't have type bool.  An incorrect parse of
  // "(a ?? b) || c" would allow b to have any type provided that a is bool.
  false ?? 1 || true; //# 06: compile-time error

  // "a || b ?? c" should parse as "(a || b) ?? c", therefore it is a static
  // type warning if b doesn't have type bool.  An incorrect parse of
  // "a || (b ?? c)" would allow b to have any type provided that c is bool.
  false || 1 ?? true; //# 07: compile-time error

  // An incorrect parse of "a || (b ?? c)" would result in no checked-mode
  // error.
  Expect.throwsAssertionError(() => false || null ?? true);
}