[committed] libstdc++: Define __cpp_lib_constexpr_algorithms in <utility> (LWG 3792)

[Jonathan Wakely <jwakely@redhat.com>]

Tested powerpc64le-linux, pushed to trunk.

-- >8--

We actually defined this macro in <utility> at one point, but I removed
it in r10-7901-g2025db692e9ed1.

libstdc++-v3/ChangeLog:

	* include/std/utility (__cpp_lib_constexpr_algorithms): Define,
	as per LWG 3792.
	* testsuite/20_util/exchange/constexpr.cc: Check for it.
---
 libstdc++-v3/include/std/utility                     | 4 ++++
 libstdc++-v3/testsuite/20_util/exchange/constexpr.cc | 6 ++++++
 2 files changed, 10 insertions(+)

diff --git a/libstdc++-v3/include/std/utility b/libstdc++-v3/include/std/utility
index 43d8765228c..006b19c00fc 100644
--- a/libstdc++-v3/include/std/utility
+++ b/libstdc++-v3/include/std/utility
@@ -86,6 +86,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #if __cplusplus >= 201402L
 #define __cpp_lib_exchange_function 201304L
 
+#if __cplusplus > 201703L
+# define __cpp_lib_constexpr_algorithms 201806L
+#endif
+
   /// Assign @p __new_val to @p __obj and return its previous value.
   template <typename _Tp, typename _Up = _Tp>
     _GLIBCXX20_CONSTEXPR
diff --git a/libstdc++-v3/testsuite/20_util/exchange/constexpr.cc b/libstdc++-v3/testsuite/20_util/exchange/constexpr.cc
index 6c130f772f8..2ec13044681 100644
--- a/libstdc++-v3/testsuite/20_util/exchange/constexpr.cc
+++ b/libstdc++-v3/testsuite/20_util/exchange/constexpr.cc
@@ -20,6 +20,12 @@
 
 #include <utility>
 
+#ifndef __cpp_lib_constexpr_algorithms
+# error "Feature test macro for constexpr std::exchange is missing in <utility>"
+#elif __cpp_lib_constexpr_algorithms < 201806L
+# error "Feature test macro for constexpr std::exchange has wrong value in <utility>"
+#endif
+
 constexpr bool
 test()
 {
-- 
2.39.2