If you have generic parameters <T, U> and requirements of the form:
- T : P
- T == ConcreteType<U>
- T.[P]U : SomeClass
- T.[P]U : SomeProto
And furthermore SomeClass does not conform to SomeProto, we can't leave
`T.[P]U : SomeClass` unsubstituted; we still have to replace `T` with
`ConcreteType<U>` to transform the latter two requirements into:
- U : SomeClass
- U : SomeProto
"Concrete contraction" is easily the hackiest part of the Requirement
Machine; I need to come up with a more principled solution for the
problem that it solves sooner or later.
Fixes rdar://problem/94150249.
We resolve protocol typealiases to DependentMemberTypes because the
Requirement Machine treats them as rewrite rules. However, it doesn't
do this for typealiases in protocol extensions.
Fixes rdar://problem/94150249.
