Incorrect downwards type inference in CFE

[munificent]

OK, I believe CFE is doing the wrong thing here, but it's in a corner of type inference I don't understand well, so I could be wrong. For what it's worth, analyzer does not report an error on the same code, so one of our tools is wrong:

class C<T> {
  foo<E extends T>(List<E> list) {
    List<E> variable = method(list);
  }

  List<F> method<F extends T>(List<F> list) => list;
}

In CFE, this produces:

../../../Documents/temp.dart:3:31: Error: The argument type 'List<E>' can't be assigned to the parameter type 'List<Never>'.
 - 'List' is from 'dart:core'.
    List<E> variable = method(list);

It looks like CFE is inferring List<Never> from the downwards inference (I'm guessing by taking the greatest/least closure to eliminate the inferred E extends T and only after that is done looking at the parameter's type.

@leafpetersen @matanlurey

[matanlurey]

Worth mentioning: This did not occur prior to null-safety opt-in.

[leafpetersen]

Yeah, this looks like a bug to me. We're inferring the call method(list) in context List<E>. That implies a downwards inference constraint solve of List<F> <# List<E> solving for F. That reduces to F <# E solving for F, which produces the constraint _ <: F <: E. With no further constraints, we should fix F to be E. That's a valid solution, so there should be no further issues

[johnniwinther]

@stefantsov Can you take a look?