Returning a FutureOr in an asynchronous function fails

[eernstg]

Consider the following program:

import 'dart:async';
FutureOr<int> foo() => 42;
Future<num> bar() async => foo();
main() => bar();

The analyzer in strong mode responds as follows (when the program is stored in n020.dart):

> dartanalyzer --version
dartanalyzer version 2.0.0-dev.26.0
> dartanalyzer --strong n020.dart
Analyzing n020.dart...
  error • The return type 'Future<FutureOr<int>>' isn't a 'Future<num>', as defined by the method 'bar' at n020.dart:5:28 • return_of_invalid_type
1 error found.

Presumably, this means that the returned expression in bar is checked in a manner which does not precisely correspond to the flatten function (from the language specification, sections Function Expressions and Return):

If \code{$T =$ FutureOr<$S$>} then $flatten(T) = S$.
...
It is a static type warning if the body of $f$ is marked \ASYNC{} and
the type \code{Future<$flatten(T)$>} (\ref{functionExpressions}) may not
be assigned to the declared return type of $f$.

But Future<int> may be assigned to Future<num> so the program should be accepted. Note that analysis of the same program succeeds with dartanalyzer --preview-dart-2.

[leafpetersen]

Probably related: #30638