NNBD: throw in async function literal causing incorrect return type

[jakemac53]

You should be able to pass a function which always throws which can masquerade as having essentially any return type, but currently that is not possible.

Example:

void main() async {
  Future<String> Function(int) x = (int v) async {
    throw v;
  }
}

Gives the following error:

 Error: A value of type 'Future<FutureOr<String>> Function(int)' can't be assigned to a variable of type 'Future<String> Function(int)'.
 - 'Future' is from 'dart:async'.
 - 'FutureOr' is from 'dart:async'.
  Future<String> Function(int) x = (int v) async {

It looks like it is inferring the type FutureOr<String> when it would need to be Future<Never> or possibly Future<String>.

If you extract the function literal out to a top level method it is fine:

void main() async {
  Future<String> Function(int) y = x;
}

Future<String> x(int v) async {
  throw v;
}

This appears to only be a problem in the CFE and not analyzer.

cc @natebosch

[johnniwinther]

I cannot reproduce. What command line do you use to run the repo and with what version?

[jakemac53]

In an opted in package: dart --enable-experiment=non-nullable --null-safety <file.dart>

Edit: the --null-safety flag is not required

[johnniwinther]

It only reproduces when written like this:

void main() async {
  Future<String> Function(int) x = (int v) async {
    throw v;
  };
}

But now I've got something to go on!

[jakemac53]

I must have been trying to shorten the repro and forgot to test that version sorry! I updated the original also.