It certainly shouldn't segfault (please file a bug at https://bugs.swift.org!) but the problem here is that any subclasses of Game would have a different type. That is, it's not obvious that a PassthroughSubject<CardGame, Never> is a kind of PassthroughSubject<Game, Never>, even though CardGame is a kind of Game. This feature is called "covariant generic parameters" and would be non-trivial to add to Swift.
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