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

["wjwray at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

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

Will Wray <wjwray at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |wjwray at gmail dot com

--- Comment #1 from Will Wray <wjwray at gmail dot com> ---
I'm keen to see this fixed (and open to contribute)
(defaulted array comparison now works on Clang & MSVC).

Workaround for lack of compiler-generated array<=>array
is awkward and brittle: https://godbolt.org/z/xr668E

 * Preprocessor conditional compilation is required
   (at least there seems no way to detect array<=>array
    support and dispatch to a user-defined comparison
    only as needed - iff array members are present):

    # if ! defined(__GNUC__) || defined(__clang__)
        auto operator<=>(C const&) const = default;
    # else
        constexpr auto operator<=>(C const& r) const {
            return three_way_compare(x,r.x);
        }
    #endif

Then:
 * A generic 3-way comparison for array should be recursive.
 * Achieving efficient/vector codegen is not straightforward.
 * Deducing the return type is subtle:

    template <typename E, int N>
    constexpr auto three_way_compare(E const(&l)[N],
                                     E const(&r)[N]) {
        auto c = l[0] <=> r[0];
        for (int i = 0; ++i != N; c = l[i] <=> r[i])
            if (c != 0)
                return c;
        return c;
    }

So it'd be better for all if this were compiler-generated.