libstdc++: Simplify constraints on std::compare_three_way

The __3way_builtin_ptr_cmp concept can use three_way_comparable_with to
check whether <=> is valid. Doing that makes it obvious that the
disjunction on compare_three_way::operator() is redundant, because
the second constraint subsumes the first.

The workaround for PR c++/91073 can also be removed as that bug is fixed
now.

	* libsupc++/compare (__detail::__3way_builtin_ptr_cmp): Use
	three_way_comparable_with.
	(__detail::__3way_cmp_with): Remove workaround for fixed bug.
	(compare_three_way::operator()): Remove redundant constraint from
	requires-clause.
	(__detail::_Synth3way::operator()): Use three_way_comparable_with
	instead of workaround.
	* testsuite/18_support/comparisons/object/93479.cc: Prune extra
	output due to simplified constraints on compare_three_way::operator().

libstdc++-v3/ChangeLog

 2020-01-29  Jonathan Wakely  <jwakely@redhat.com>
 
+	* libsupc++/compare (__detail::__3way_builtin_ptr_cmp): Use
+	three_way_comparable_with.
+	(__detail::__3way_cmp_with): Remove workaround for fixed bug.
+	(compare_three_way::operator()): Remove redundant constraint from
+	requires-clause.
+	(__detail::_Synth3way::operator()): Use three_way_comparable_with
+	instead of workaround.
+	* testsuite/18_support/comparisons/object/93479.cc: Prune extra
+	output due to simplified constraints on compare_three_way::operator().
+
 	PR libstdc++/93479
 	* libsupc++/compare (__3way_builtin_ptr_cmp): Require <=> to be valid.
 	* testsuite/18_support/comparisons/object/93479.cc: New test.

libstdc++-v3/libsupc++/compare

@@ -525,26 +525,20 @@ namespace std
     // BUILTIN-PTR-THREE-WAY(T, U)
     template<typename _Tp, typename _Up>
       concept __3way_builtin_ptr_cmp
-	= requires(_Tp&& __t, _Up&& __u)
-	    { static_cast<_Tp&&>(__t) <=> static_cast<_Up&&>(__u); }
+	= three_way_comparable_with<_Tp, _Up>
 	  && convertible_to<_Tp, const volatile void*>
 	  && convertible_to<_Up, const volatile void*>
 	  && ! requires(_Tp&& __t, _Up&& __u)
 	     { operator<=>(static_cast<_Tp&&>(__t), static_cast<_Up&&>(__u)); }
 	  && ! requires(_Tp&& __t, _Up&& __u)
 	     { static_cast<_Tp&&>(__t).operator<=>(static_cast<_Up&&>(__u)); };
-
-    // FIXME: workaround for PR c++/91073
-    template<typename _Tp, typename _Up>
-      concept __3way_cmp_with = three_way_comparable_with<_Tp, _Up>;
   } // namespace __detail
 
   // [cmp.object], typename compare_three_way
   struct compare_three_way
   {
     template<typename _Tp, typename _Up>
-      requires (__detail::__3way_cmp_with<_Tp, _Up>
-	  || __detail::__3way_builtin_ptr_cmp<_Tp, _Up>)
+      requires three_way_comparable_with<_Tp, _Up>
       constexpr auto
       operator()(_Tp&& __t, _Up&& __u) const noexcept
       {
@@ -919,7 +913,7 @@ namespace std
 	  { __u < __t } -> convertible_to<bool>;
 	}
 	{
-	  if constexpr (__3way_cmp_with<_Tp, _Up>)
+	  if constexpr (three_way_comparable_with<_Tp, _Up>)
 	    return __t <=> __u;
 	  else
 	    {

libstdc++-v3/testsuite/18_support/comparisons/object/93479.cc

@@ -42,3 +42,5 @@ test02()
   std::compare_three_way{}(x, ""); // { dg-error "no match" }
   std::compare_three_way{}("", x); // { dg-error "no match" }
 }
+
+// { dg-prune-output "in requirements with" }