LanguageFeatures/Generic-functions-as-type-args/function_A01_t05.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 statically that generic function can be a function type
 * argument and bound.
 * @author iarkh@unipro.ru
 */
// SharedOptions=--enable-experiment=generic-metadata

typedef exp1 = T Function<T extends num>(T);
typedef exp2 = void Function<T extends num>();
typedef exp3 = T Function<T extends num>();
typedef exp4 = void Function<T extends num>(T);

void func1<T extends T Function<T extends num>(T)>() {}

void func2<T extends void Function<T extends num>()>() {}

void func3<T extends T Function<T extends num>()>() {}

void func4<T extends void Function<T extends num>(T)>() {}

main() {
  func4<exp1>();

  func4<exp2>();
//      ^
// [analyzer] unspecified
// [cfe] unspecified

  func4<exp3>();
//      ^
// [analyzer] unspecified
// [cfe] unspecified

  func4<exp4>();
}