test/Compatibility/self_same_type.swift

// RUN: %target-typecheck-verify-swift -swift-version 3

// Note: while Swift 3.2 originally intended to provide backward
// compatibility here, the type-soundness issue was considered so severe
// (due to it breaking the optimizer) that that we escalated it to an
// error.

protocol P {
  associatedtype T
}

protocol Q {
  func foo<T: P>(_: T, _: T.T) where T.T == Self
}

class C1: Q {
  func foo<T: P>(_: T, _: C1) where T.T == C1 {} // expected-error{{instance method 'foo' in non-final class 'C1' cannot be used to satisfy requirement instance method 'foo' (in protocol 'Q') due to same-type requirement involving 'Self'}}}}
    // expected-note@-1{{consider weakening the same-type requirement 'T.T' == 'C1' to a superclass requirement}}{{41-43=:}}
}

final class C2: Q {
  func foo<T: P>(_: T, _: C2) where T.T == C2 {}
}

class C3: Q {
  func foo<T: P>(_: T, _: C3) where T.T: C3 {}
}