class ReadOnly { } class ReadWrite: ReadOnly { } protocol IO { associatedtype Manifest: ReadOnly = ReadOnly } func foo<P>(_ arg: P) where P: IO, P.Manifest: ReadWrite { // ReadWrite } class UserDefaults: IO { } foo(UserDefaults()) // 🙅 (edited)class ReadOnly { } class ReadWrite: ReadOnly { } protocol IO { associatedtype Manifest: ReadOnly = ReadOnly } func foo<P>(_ arg: P) where P: IO, P.Manifest: ReadWrite { // ReadWrite } class UserDefaults: IO { } foo(UserDefaults()) // 🙅 (edited)