そういえばDigital Loungesでこんなやりとりありましたねー Thanks! So writing @available(*, unavailable) extension X: Sendable means that a type is explicitly not thread safe? Yes, exactly. The compiler treats an explicitly unavailable Sendable conformance as a stronger sign that the type is not thread-safe, so it may treat them slightly differently from ones that are simply missing—for instance, it may show warnings it would normally suppress, or phrase them differently, or suggest different Fix-Its. (edited)