Interesting, thank you for the example. I'm mostly used to how Rust handles this, and in its approach individual items such as functions need to be "standalone sane".

    func foo[T any, V any](x T) V {
        return x.y
    }

would also not fly there, because T and V are not usefully constrained to anything. Go is the same then. I prefer that model, as it makes local reasoning that much more robust. The C++ approach is surprising to me, never would have thought that's possible. It seems very magic.