Inconsistent inference in analyzer and CFE

[leafpetersen]

The following code is inferred inconsistently between the CFE and the analyzer:

void main(){
  String? variable;
  var f = Future.value(variable);
  Future<String> x = f;
}

The analyzer emits no error, suggesting that inference is inferring String for the type argument to Future.value, whereas the CFE emits the following error:

leafp-macbookpro2:sdk leafp$ ~/src/dart-repo/sdk/xcodebuild/ReleaseARM64/dart ~/tmp/example.dart
../../../tmp/example.dart:5:22: Error: A value of type 'Future<String?>' can't be assigned to a variable of type 'Future<String>' because 'String?' is nullable and 'String' isn't.
 - 'Future' is from 'dart:async'.
  Future<String> x = f;

I believe the analyzer behavior here is in general more useful, I'm not sure off hand which is consistent with the spec. cc @stereotype441 @eernstg @johnniwinther @scheglov

[scheglov]

We apply following rules when gathering constraints here in the analyzer:

String? vs FutureOr<T>?

• If Q is Q0? the match holds under constraint set C:
  • If P is P0? and P0 is a subtype match for Q0 under constraint set C.

String vs FutureOr<T>

• If Q is FutureOr<Q0> the match holds under constraint set C:
  • Or if P is a subtype match for Q0 under constraint set C.

String vs T.

[johnniwinther]

cc @chloestefantsova

[lrhn]

This does suck extra because the parameter to Future.value is optional and nullable, and it's not part of the type system that you must only pass null when the type argument is nullable.

(We should fix that!)