Implement LWG-3746 optional's spaceship with U with a type derived from optional causes infinite constraint meta-recursion

[A. Jiang (frederick-vs-ja)]

Fixes #3225.

I found that MSVC's behavior is strange during testing (Godbolt link), so currently I haven't add the original example yet.

#include <optional>

struct derived_optional : std::optional<int> {
    friend bool operator==(const derived_optional&, const derived_optional&) = default;
    friend auto operator<=>(const derived_optional&, const derived_optional&) = default;
};

int main()
{
    derived_optional a;
    std::optional<derived_optional> b;
#ifdef LESS // If b < a is not skipped, then MSVC accepts b <=> a.
    (void) (b < a);
#endif
    (void) (b <=> a);
}

[Stephan T. Lavavej (StephanTLavavej)]

No change requested: I observe that these temporaries could be constructed with braces, following our preferred modern convention. Although this test uses braces to construct temporaries elsewhere, I can't quite bring myself to reset testing for such a small issue in test code (which certainly doesn't affect correctness or test coverage).

[Stephan T. Lavavej (StephanTLavavej)]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[Stephan T. Lavavej (StephanTLavavej)]

🛸 ♾️ 🔁