From 6c22fe418cff57dad712c4b950638e6e2d09bd9c Mon Sep 17 00:00:00 2001
From: Jonathan Wakely <jwakely@redhat.com>
Date: Wed, 24 Jul 2024 11:32:22 +0100
Subject: [PATCH] libstdc++: Fix <ostream> and <istream> for -std=gnu++14
-fconcepts [PR116070]
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 11d51d3e666c..a2b207dae78c 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 8a21758d0a33..12be6c4fd178 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>
--
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