Incorrect implicit downcast error in async functions with FutureOr

[leafpetersen]

The following code, when analyzer with --strong --no-implicit-casts produces type errors on both function returns:

import 'dart:async';
typedef FutureOr<String> Stuff();
Stuff stuff = () async => stuff2();
Future<String> stuff3() async => stuff2();
Stuff stuff2;

[error] The return type 'Future<FutureOr<String>>' isn't a 'Future<String>', as defined by the method ''. (/Users/leafp/tmp/ddctest.dart, line 5, col 27)
[error] The return type 'Future<FutureOr<String>>' isn't a 'Future<String>', as defined by the method 'stuff3'. (/Users/leafp/tmp/ddctest.dart, line 7, col 34)

As far as I can tell, there should be no error here: there is no implicit downcast.

Oddly enough, DDC (correctly) does not insert a downcast, so this error must presumably be coming from somewhere other than the checker.

cc @natebosch @jmesserly @stereotype441

[jmesserly]

It's StaticWarningCode.RETURN_OF_INVALID_TYPE from ErrorVerifier, most likely _checkForReturnOfInvalidType. I thought we'd disabled most of ErrorVerifier type checking in strong mode (because CodeChecker in checker.dart supersedes it), but I guess we missed some checks. It probably does not understand FutureOr<T>

[leafpetersen]

Possibly related example from @nex3

import 'dart:async';

bool c = true;

Future<void> test() async {
  Future<int> a = new Future.value(3);
  Future<dynamic> b = new Future<dynamic>.value(3);
  if (c) {
    return a;
  } else {
    return b;
  }
}

Currently this produces an error on the first return but not the second.

[leafpetersen]

Another example reported in #31398

import 'dart:async';

Future<int> fn(int interpolation) async {
  return runCallback(() => 1);
}

T runCallback<T>(T callback()) {
  return callback();
}