[Bug c++/93480] Defaulted <=> doesn't expand array elements

[wjwray at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93480

--- Comment #2 from Will Wray <wjwray at gmail dot com> ---
For reference, here's a macro-free workaround to provide portable
operator<=> for templated classes with array members, defaulting
where possible (current Clang and MSVC) otherwise dispatching to
a user-defined implementation (current gcc)
https://godbolt.org/z/qbEfh7

First, a trait to check default 3-way array compare:

    inline constexpr bool has_default_array_compare = [] {
        struct C {
            char c[1];
            auto operator<=>(C const&) const = default;
        };
        return std::three_way_comparable<C>;
    }();

Then constrain the operator<=> definitions

    template <typename T, int N> struct array
    {
        T data[N];

        friend auto operator<=>(array const&, array const&)
            requires has_default_array_compare = default;

        friend constexpr auto operator<=>(array const& l,
                                          array const& r)
            requires (!has_default_array_compare)
        {
            return compare_three_way{}(l.data,r.data);
        }
    };

However, this only works for templated classes
as it is not (yet) allowed to constrain non-template functions.