Failures on [cfe] Implement new async return rules

[johnniwinther]

There are new test failures on [cfe] Implement new async return rules.

The tests

language/unsorted/flatten_test/12 CompileTimeError (expected Pass)
language/unsorted/flatten_test/12 CompileTimeError (expected RuntimeError)

are failing on configurations

cfe-strong-linux
cfe-weak-linux
dartdevk-strong-linux-release-chrome
dartdevk-weak-linux-release-chrome
dartk-strong-linux-debug-x64
dartk-strong-linux-release-x64
dartk-weak-asserts-linux-debug-x64
dartk-weak-asserts-linux-release-x64

[johnniwinther]

@eernstg Is language/unsorted/flatten_test/12 still valid given the new rules for return async?

[eernstg]

This involves function literal return type inference:

The context type for a function literal in an invocation like ((...) async => e)() is empty, so inference for e is done with the empty imposed return type schema _. FutureOr<futureValueTypeSchema(_)> == FutureOr<_> is used as the context type for e, but in this case the type of e will be Divergent<int> in any case.

So the 'actual returned type' of the literal is UP(flatten(Divergent<int>), Never) == Divergent<Divergent<int>>, the inferred return type of the function literal is Future<flatten(Divergent<Divergent<int>>)> == Future<Divergent<Divergent<Divergent<int>>>> and the future value type is Divergent<Divergent<Divergent<int>>>.

In order to be allowed to return an expression of type Divergent<int> we must have that Divergent<int> is assignable to the future value type (not so), or flatten(Divergent<int>) == Divergent<Divergent<int>> is a subtype of the future value type (also not true).

So I believe that subtest 12 is a compile-time error today.

Cf. dart-lang/language#1057.