Soundness issue with generator element type

[eernstg]

Thanks to @lrhn for spotting this issue and reporting it here.

The specification of the element type of a generator has just been updated accordingly, cf. dart-lang/language#3218.

The old rule gave rise to a soundness issue:

import 'dart:async';

FutureOr<Iterable<int>> f() sync* {
  yield 'Hello!' as dynamic;
}

void main() {
  var iterable = f();
  if (iterable is Future<Object?>) return;
  int i = iterable.first;
  print(i); // 'Hello!' is not an `int`.
}

The main idea in the correction is that the element type of a generator function uses a reduction step where the given return type is simplified to an Iterable<T> for some T (for a sync* function body) or a Stream<T> for some T (for an async* body), and the element type is then extracted from that Iterable<T> / Stream<T>.

Subtasks:

• [cfe] Soundness issue with generator element type #53053
• [analyzer] Soundness issue with generator element type #53054

[chloestefantsova]

Sorry, I accidentally closed this one instead of #53053. Reopened it again. Thanks for noticing, @eernstg!