[committed 2/2] libstdc++: Fix <ostream> and <istream> for -std=gnu++14 -fconcepts [PR116070]

[Jonathan Wakely]

Tested x86_64-linux.

Pushed to trunk. Backports to follow after the 14.2 release.

-- >8 --

This questionable combination of flags causes a number of errors. The
ones in the rvalue stream overloads need to be fixed in the gcc-14
branch so I'm committing it separately to simplify backporting.

libstdc++-v3/ChangeLog:

	PR libstdc++/116070
	* include/std/istream: Check feature test macro before using
	is_class_v and is_same_v.
	* include/std/ostream: Likewise.
---
 libstdc++-v3/include/std/istream | 2 +-
 libstdc++-v3/include/std/ostream | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/istream b/libstdc++-v3/include/std/istream
index 11d51d3e666..a2b207dae78 100644
--- a/libstdc++-v3/include/std/istream
+++ b/libstdc++-v3/include/std/istream
@@ -1069,7 +1069,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // 2328. Rvalue stream extraction should use perfect forwarding
   // 1203. More useful rvalue stream insertion
 
-#if __cpp_concepts >= 201907L
+#if __cpp_concepts >= 201907L && __glibcxx_type_trait_variable_templates
   template<typename _Is, typename _Tp>
     requires __derived_from_ios_base<_Is>
       && requires (_Is& __is, _Tp&& __t) { __is >> std::forward<_Tp>(__t); }
diff --git a/libstdc++-v3/include/std/ostream b/libstdc++-v3/include/std/ostream
index 8a21758d0a3..12be6c4fd17 100644
--- a/libstdc++-v3/include/std/ostream
+++ b/libstdc++-v3/include/std/ostream
@@ -768,7 +768,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // _GLIBCXX_RESOLVE_LIB_DEFECTS
   // 1203. More useful rvalue stream insertion
 
-#if __cpp_concepts >= 201907L
+#if __cpp_concepts >= 201907L && __glibcxx_type_trait_variable_templates
   // Use concepts if possible because they're cheaper to evaluate.
   template<typename _Tp>
     concept __derived_from_ios_base = is_class_v<_Tp>
-- 
2.45.2