LanguageFeatures/Super-bounded-types/static_analysis_A01_t08.dart

// Copyright (c) 2018, 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 We say that the parameterized type G<T1..Tk> is regular-bounded
/// when Tj <: [T1/X1 .. Tk/Xk]Bj for all j, 1 <= j <= k, where X1..Xk are the
/// formal type parameters of G in declaration order, and B1..Bk are the
/// corresponding upper bounds.
/// This means that each actual type argument satisfies the declared upper bound
/// for the corresponding formal type parameter.
/// We extend covariance for generic class types such that it can be used also in
/// cases where a type argument violates the corresponding bound.
/// For instance, assuming the classes [C] and [D] as declared in the
/// [Motivation] section, [C<D>] is a subtype of [C<Object>]. This is new because
/// [C<Object>] used to be a compile-time error, which means that no questions
/// could be asked about its properties. Note that this is a straightforward
/// application of the usual covariance rule: [C<D> <: C<Object>] because
/// [D <: Object]. We need this relaxation of the rules in order to be able to
/// define which violations of the declared bounds are admissible.
/// @description Checks that assigning [Object] as a bound of super-bounded type
/// does not cause compile error in case of several type parameters.
/// @author iarkh@unipro.ru
/// @issue #32903


class C<X extends C<X, Y>, Y extends C<X, Y>> {}
class D extends C<D, D> {}

main() {
  D d = new D();
  C<Object?, Object?> c = d;
  dynamic c2 = d;
}