AbstractDefChecker doesn’t resolve free variables in return type

[kimburgess]

When declaring an abstract method that has a self-referencing outer type, but free inner such as:

module Functor(T)
  abstract def fmap(&block : T -> U) : Functor(U) forall U
end

A compiler warning - Warning: can't resolve return type Functor(U) - is generated.

It appears that an additional resolution step may be required for free variables before flagging issues.

[Blacksmoke16]

FWIW this won't actually compile anymore due to #9810.

[icy-arctic-fox]

The example code now produces an error - tested with Crystal 1.9.2

It appears that free variables do not work in the return type of abstract defs.