https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98842
g...@nicholas-schwab.de changed:
What |Removed |Added
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CC| |g...@nicholas-schwab.de
--- Comment #1 from g...@nicholas-schwab.de ---
Three problem lies in the concept three_way_comparable_with. It uses
__detail::__weakly_eq_cmp_with that requires u == t. Due to paper P1185
operator== will not lookup operator<=>. Therefore having only operator<=>
defined does not suffice to fulfill __detail::__weakly_eq_cmp. Hence the first
of these two templates is SFINAE'D out and the second is taken.
template<typename _Tp, three_way_comparable_with<_Tp> _Up>
constexpr compare_three_way_result_t<_Tp, _Up>
operator<=>(const optional<_Tp>& __x, const optional<_Up>& __y)
template<typename _Tp, typename _Up>
constexpr compare_three_way_result_t<_Tp, _Up>
operator<=>(const optional<_Tp>& __x, const _Up& __v)
The second will however compare false whenever the left hand side is empty.
The standard requires that three_way_comparable_with also requires
weakly_equality_comparable. Hence the implementation of this concept is
correct. However this might be a defect in the standard.
Note that the standard also wants three_way_comparable_with<_Tp> for the second
template (http://eel.is/c++draft/optional#comp.with.t-25). So the second
template above is non-confirming. But it seems to me that a comparison between
two optionals then is always ambiguous, so this should be a defect.
