Future.wait([f1, f2]) does not have the correct return type

[scheglov]

This code has a warning WARNING: Unsound implicit cast from List<dynamic> to List<List<A>> ([my_test] bin/test2.dart:6).

import 'dart:async';

foo() async {
  Future<List<A>> f1 = null;
  Future<List<A>> f2 = null;
  List<List<A>> merged = await Future.wait([f1, f2]);
}

class A {}

but if I extract [f1, f2] into a local variable, it does not have this warning.

import 'dart:async';

foo() async {
  Future<List<A>> f1 = null;
  Future<List<A>> f2 = null;
  var futures = [f1, f2];
  List<List<A>> resultsList = await Future.wait(futures);
}

class A {}

[jmesserly]

other workarounds, you could also put an explicit type on the list <Future<List<A>>>[f1, f2] or on the generic method var resultsList = await Future.wait/*<List<A>>*/(futures);.

but yeah, this should work. What should happen is await creates a T | F<T> downwards context, then generic method downward inference picks the F<T> branch because the return type of Future.wait is a Future<List>. However I'm not sure if either of those steps are currently implemented. I'm fairly sure the second one is not.

edit: fix what I wrote about Future.wait's type signature

[jmesserly]

the high level issue here is https://github.com/dart-lang/sdk/issues/25490, that downwards and upwards inference aren't unified. It's possible we push down List<Future<dynamic>> as the context for Future.wait's argument type. If we do that then it probably prevents the upwards ListLiteral inference that could otherwise easily figure out the list type argument (and thus, the generic method call on the way back up).

[jmesserly]

Guess above was correct. It was downwards generic method inference dropping the Future<T> possibility from T | Future<T>. Fixed in this CL: https://codereview.chromium.org/2280463002/

[scheglov]

See also #27151.