Dartanalyzer does not allow declare class A with "X extends FutureOr<A<X>>" type parameter .

[iarkh]

Dart SDK Version: 2.1.0-dev.2.0
OS: Windows 10 (64 bit)

The following code sample declares two classes with Future and FutureOr type parameters:

import "dart:async";

class B<X extends Future<A<X>>> {}
class A<X extends FutureOr<A<X>>> {}

Dart passes with both code lines, i.e. just print OK as a result.

Dartanalyzer passes with class B<X extends Future<A<X>>> {} and fails with class A<X extends FutureOr<A<X>>> {}, sample output is:

Analyzing test.dart...
  error - 'X' doesn't extend 'FutureOr<A<X>>' at test.dart:4:30 - type_argument_not_matching_bounds
1 error found.

I believe this is incorrect result and analyzer should pass both for Future and FutureOr parameters.

[askeksa-google]

The spec says (in 10.9):
"It is a compile-time error if the type in the extends clause of a class C is a type variable (14), a type alias (19.3.1), an enumerated type (13), a malformed type (19.1), a deferred type (19.1), type dynamic (19.6), or type FutureOr<T> for any T (19.7)."
So the class declaration with FutureOr as supertype is not valid Dart. See also #33744

[eernstg]

Oh, but in this case we do not use FutureOr<..> as a superclass, we use it as the bound of a type variable (in this case: an F-bounded one), and I couldn't spot any reason why that would be an error.

It's not even impossible to find a type argument that satisfies this constraint: A<Null> would do, and with class C extends A<C> {}, A<C> would do as well (C can be given as an actual type argument to A in the first place because C <: A<C> <: FutureOr<A<C>>), and similarly for class D extends Future<A<D>>> {} and A<D>.

[iarkh]

And anyway, analyzer and dart should behave in the same way here.

[eernstg]

Closing: dart as well as dartanalyzer accepts the example program as of a49217b.