Can't return a value from a generator function that uses the 'async*' or 'sync*' modifierDRT-W1602

The analyzer produces this diagnostic when a generator function (one whose body is marked with either async* or sync*) uses either a return statement to return a value or implicitly returns a value because of using =>. In any of these cases, they should use yield instead of return.

Examples

The following code produces this diagnostic because the method f is a generator and is using return to return a value:

Iterable<int> f() sync* {
  return 3;
}

The following code produces this diagnostic because the function f is a generator and is implicitly returning a value:

Stream<int> f() async* => 3;

Common fixes

If the function is using => for the body of the function, then convert it to a block function body, and use yield to return a value:

Stream<int> f() async* {
  yield 3;
}

If the method is intended to be a generator, then use yield to return a value:

Iterable<int> f() sync* {
  yield 3;
}

If the method isn't intended to be a generator, then remove the modifier from the body (or use async if you're returning a future):

int f() {
  return 3;
}