Incorrect behavior of return in async function.

[lrhn]

The Dart 2 behavior for returns in async functions is to await a future if it's a future of the flattened return type.

That is:

Future<int> foo() { 
   return someExpression;
}

will evaluate someExpression with context type FutureOr<int>, and if it is a Future<int>, it is awaited before returning its value.

That it's awaited and not passed directly to a completer (an implementation detail) is important in the case where the return is inside a try/catch an you try to return a future containing an error.

Example:

Future<int> foo() {
   try {
     return compute();
   } catch (e) {
     return -1;
   }
}
Future<int> compute() async { 
  throw "No";
}
main() {
 foo().then(print);  // should print -1
}

See #22730 for history of the spec change.

[lrhn]

Another reason awaiting is important is code like:

Future<int> foo() async {
  try {
    return Future<int>.error("gotcha");
  } finally {
    return 0;
  }
}

The issue here is that if the returned future is not awaited at the return statement, it will be silently discarded and the error becomes uncaught instead of thrown.

[natebosch]

It's not clear to me what the concrete actions are to resolve this. Can we describe the specific change we are hoping for in the implementations?

Should we consider pushing on dart-lang/language#870 to fix the ambiguity?

[stereotype441]

@lrhn @eernstg Is this a duplicate of #44395?

[eernstg]

Yes, they are very similar, so #44395 is essentially a duplicate of this one. I didn't find this one when I created #44395.

[stereotype441]

Duplicate of #44395