Can mixed_return_types be relaxed for async functions that return Future<void>?

[jamesderlin]

Given the following code:

Future<void> _cachedFuture;

Future<void> foo(String arg) async {
  if (arg.isEmpty) {
    return;
  }

  if (_cachedFuture != null) {
    return _cachedFuture;
  }

  _cachedFuture = Future.delayed(const Duration(seconds: 1));
  await _cachedFuture;
  print('Done');
}

Future<void> main() async {
  await foo('');
  await foo('xyzzy');
}

dartanalyzer will complain about the return; line:

error • Functions can't include return statements both with and without values. • return_future.dart:7:5 • mixed_return_types
error • Functions can't include return statements both with and without values. • return_future.dart:11:5 • mixed_return_types
2 errors found.

This seems overzealous for an async function declared to return Future<void> given that:

Future<void> f() async {
  return;
}

Future<void> g() async {
  return f();
}

are legal and since the analyzer doesn't complain about not having a return statement when reaching the end of the function.

To avoid the analysis error, I need to replace return; with return Future.value(), which I think is less clear.

[eernstg]

There is no rule that returns cannot be mixed like this, but we used to have such a rule. Cf. #39476.

[eernstg]

I believe there is no connection to the linter in this issue, and the SDK issue has been reported, so I'll close this issue.