Future produced by async function has wrong type argument

[mraleph]

Originally posted by @TimWhiting in #49345 (comment)

A future related test is failing on flutter/dart master with the fpdart library, found while creating this pull request to fix mixin breaking changes to work on master. I believe I'm running with a new enough sdk to incorporate the soundness fix.

SandroMaglione/fpdart#42

This is the test.

 test('run', () async {
      final task = Task.of(10);
      final future = task.run();
      expect(future, isA<Future<int>>());
      final r = await future;
      expect(r, 10);
 });

This is the relevant bit of Task

/// [Task] represents an asynchronous computation that yields a value of type `A` and **never fails**.
///
/// If you want to represent an asynchronous computation that may fail, see [TaskEither].
class Task<A>  {
  final Future<A> Function() _run;

  /// Build a [Task] from a function returning a [Future].
  const Task(this._run);

  /// Build a [Task] that returns `a`.
  factory Task.of(A a) => Task<A>(() async => a);

  /// Run the task and return a [Future].
  Future<A> run() => _run();
}

dart version:

dart --version
Dart SDK version: 2.18.0-263.0.dev (dev) (Thu Jul 7 10:24:22 2022 -0700) on "macos_x64"

output

00:05 +558 -1: test/src/task_test.dart: Task run [E]                                                                                                                                  
  Expected: <Instance of 'Future<int>'>
    Actual: <Instance of 'Future<dynamic>'>
     Which: is not an instance of 'Future<int>'
  
  package:test_api              expect
  test/src/task_test.dart 83:7  main.<fn>.<fn>

Standalone repo with just the above code, able to reproduce:
https://github.com/TimWhiting/dart_fail_future

Analyzer shows the correct type when hovering over the future returned from Task.run()

[mraleph]

Likely related to the new async implementation.