duplicates:

• SR-7428 Class conforming to protocol with default method implementation no longer compiles with Xcode 9.3

Issue Description:

This used to compile in 4.0, but gets rejected in 4.1 and master:

protocol A {
  associatedtype B
  func foo<C: A>(_: C) where C.B == Self.B

  func bar() -> B
}

extension A {
  func foo<C: A>(_: C) where C.B == Self.B {}
}

class D: A {
  func bar() -> Int { return 0 }
}

Although it would be correct to reject a protocol extension method with a same-type constraint on Self as a witness for a non-final class, it is not correct to do so for associated types of Self, since those come from the conformance and ought to be invariant among all subclasses of a conforming type. I found this regression while attempting to build an embedded version of Realm with Xcode 9.3 as part of reproducing another crash.