libstdc++: Update __cpp_lib_three_way_comparison macro

With P1614R2 fully implemented (except for the <chrono> types which we
don't support at all) we can define the feature test macro to the new
value.

	* include/std/version (__cpp_lib_three_way_comparison): Update value.
	* libsupc++/compare (__cpp_lib_three_way_comparison): Likewise.

libstdc++-v3/ChangeLog

 2020-04-20  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/version (__cpp_lib_three_way_comparison): Update value.
+	* libsupc++/compare (__cpp_lib_three_way_comparison): Likewise.
+
 	* include/bits/stl_map.h (map): Define operator<=> and remove
 	operator< for C++20.
 	* include/bits/stl_multimap.h (multimap): Likewise.

libstdc++-v3/include/std/version

@@ -200,7 +200,7 @@
 #endif
 #define __cpp_lib_span 202002L
 #if __cpp_impl_three_way_comparison >= 201907L && __cpp_lib_concepts
-# define __cpp_lib_three_way_comparison 201711L
+# define __cpp_lib_three_way_comparison 201907L
 #endif
 #define __cpp_lib_to_array 201907L
 #endif

libstdc++-v3/libsupc++/compare

@@ -39,7 +39,7 @@
 #include <concepts>
 
 #if __cpp_lib_concepts
-# define __cpp_lib_three_way_comparison 201711L
+# define __cpp_lib_three_way_comparison 201907L
 #endif
 
 namespace std
@@ -866,12 +866,23 @@ namespace std
 
   namespace __detail
   {
-    // [expos.only.func]
+    // [expos.only.func] synth-three-way
     inline constexpr struct _Synth3way
     {
       template<typename _Tp, typename _Up>
+	static constexpr bool
+	_S_noexcept(const _Tp* __t = nullptr, const _Up* __u = nullptr)
+	{
+	  if constexpr (three_way_comparable_with<_Tp, _Up>)
+	    return noexcept(*__t <=> *__u);
+	  else
+	    return noexcept(*__t < *__u) && noexcept(*__u < *__t);
+	}
+
+      template<typename _Tp, typename _Up>
 	constexpr auto
 	operator()(const _Tp& __t, const _Up& __u) const
+	noexcept(_S_noexcept<_Tp, _Up>())
 	requires requires
 	{
 	  { __t < __u } -> __boolean_testable;
@@ -892,6 +903,7 @@ namespace std
 	}
     } __synth3way = {};
 
+    // [expos.only.func] synth-three-way-result
     template<typename _Tp, typename _Up = _Tp>
       using __synth3way_t
 	= decltype(__detail::__synth3way(std::declval<_Tp&>(),