LanguageFeatures/Generic-functions-as-type-args/typedef2_A01_t09.dart

// Copyright (c) 2021, 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.

/// @assertion Allow generic function types as type arguments and bounds
///
/// The language disallows generic function types as type arguments and bounds.
///
///  late List<T Function<T>(T)> idFunctions; // INVALID.
///  var callback = [foo<T>(T value) => value]; // Inferred as above, then invalid.
///  late S Function<S extends T Function<T>(T)>(S) f; // INVALID.
///
/// We remove that restriction, so a type argument and a bound can be a generic
/// function type.
///
/// This requires no new syntax, and in some cases only the removal of a single
/// check. There might be some platforms where the implementation currently
/// assumes that generic function types cannot occur as the value of type
/// variables (an proof-of-concept attempt hit an assert in the VM). Such
/// assumptions will need to be flushed out with tests and fixed.
///
/// Because we already infer List<T Function<T>(T)> in the code above, this
/// change will not affect type inference, it will just make the inferred type
/// not be an error afterwards.
///
/// We do not expect the removal of this restriction to affect the feasibility of
/// type inference. After all, it's already possible to have a generic function
/// type occurring covariantly in a type argument, like List<T Function<T>(T)
/// Function()>.
/// @description Checks that generic function can be an old-style function type
/// alias argument and bound.
///
/// @Issue 45313, 45718
/// @author iarkh@unipro.ru

import "../../Utils/expect.dart";

typedef void TEST<T extends void Function<TT>()>(T t);

void testme<T extends void Function<TT>()>(T t) {}
void testme1(void Function<T>()) {}

main() {
  TEST t = testme;
  TEST t1 = testme1;

  // See https://github.com/dart-lang/sdk/issues/45718 evaluation for more details
  Expect.isFalse(testme is TEST);
  Expect.isTrue(testme1 is TEST);
  Expect.runtimeIsNotType<TEST>(testme);
  Expect.runtimeIsType<TEST>(testme1);

  Expect.isTrue(t is TEST);
  Expect.isTrue(t1 is TEST);
  Expect.runtimeIsType<TEST>(t);
  Expect.runtimeIsType<TEST>(t1);
}