[committed] libstdc++: Update __cpp_lib_three_way_comparison macro

[Jonathan Wakely via Gcc-patches]

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.

Tested powerpc64le-linux, committed to master.

After committing it I noticed I'd also accidentally added a change to
__synth3way as well, which I meant to do in a separate commit. I'll
update the changelog entry to reflect that.



commit 73a0a21d22da7db9687853a4ee7d55fc9aa818a4
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Mon Apr 20 17:50:10 2020 +0100

    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.

diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index d9a47ee8e8a..6d7dcc46023 100644
--- a/libstdc++-v3/include/std/version
+++ b/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
diff --git a/libstdc++-v3/libsupc++/compare b/libstdc++-v3/libsupc++/compare
index e5fb322ed9e..28dfe843ee7 100644
--- a/libstdc++-v3/libsupc++/compare
+++ b/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&>(),