On 2020-01-04 at 15:45, Wesley Shillingford via Std-Discussion wrote:
> Dear all
>
> First time reporting something, and no idea if this is the appropriate
> way to do it.
>
> I was playing around with the three-way comparison (spaceship) operator
> and was surprised when this failed to compile with MSVC:
>
> int x{2};
> int y{2};
> auto s = x <=> y;
> s == std::strong_ordering::equal; // compile failure
>
> VC++ does not define the overload in <compare> for:
> operator==(strong_ordering, strong_ordering);
>
> I checked the standard library document and it is not defined, so VC++
> is within its right to ignore it and be confirming:
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0768r1.pdf
>
> However, gcc does define it:
> https://github.com/wezrule/gcc/blob/master/libstdc%2B%2B-v3/libsupc%2B%2B/compare#L299
>
> So this compiles with gcc and not MSVC:
> https://godbolt.org/z/uRLimE
>
> I wonder if it should be added as an alternative to comparing with 0,
> gcc (libstdc++) saw value in adding it and it would be good to not have
> yet more inconsistencies in c++ implementations.
>

A problem with implementing the standard is that the specified version
of the operator

friend constexpr bool operator==(strong_ordering v, strong_ordering w)
noexcept = default;

is not (yet) allowed - an operator can be defaulted only for
const-reference parameters, not for passed by value. :-)


Of course, there is a proposal to change this:

http://wg21.link/P1946



     Bo Persson