Incorrect "Same-type" warning (will be an error in Swift 6)

[smkuehnhold (Samuel Kuehnhold)]

I noticed another difference in behavior between functions with and without same-type constraints when opening existentials.

TLDR: Opening compiles when applying the same-type constraint to a member function of a generic type, but not otherwise.

@Slava_Pestov

In fact I would consider it a bug that this works:

class C<T: P>: Base {
  init(_: T) {}

  static func f<U: P>(_ t: U) -> Base where T == U { return C(t) }
}

But this doesn't:

class C<T: P>: Base {
  init(_: T) {}

  static func f(_ t: T) -> Base { return C(t) }
}