[committed] libstdc++: Use if-constexpr in std::__try_use_facet [PR113099]

[Jonathan Wakely]

Tested x86_64-linux. Pushed to trunk.

-- >8 --

As noted in the PR, we can use if-constexpr for the explicit
instantantiation definitions that are compiled with -std=gnu++11. We
just need to disable the -Wc++17-extensions diagnostics.

libstdc++-v3/ChangeLog:

        PR libstdc++/113099
        * include/bits/locale_classes.tcc (__try_use_facet): Use
        if-constexpr for C++11 and up.
---
 libstdc++-v3/include/bits/locale_classes.tcc | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/locale_classes.tcc 
b/libstdc++-v3/include/bits/locale_classes.tcc
index 2a6176fb4de..63097582dec 100644
--- a/libstdc++-v3/include/bits/locale_classes.tcc
+++ b/libstdc++-v3/include/bits/locale_classes.tcc
@@ -87,6 +87,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
                                __s2.data(), __s2.data() + __s2.length()) < 0);
     }
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions"
   template<typename _Facet>
     inline const _Facet*
     __try_use_facet(const locale& __loc) _GLIBCXX_NOTHROW
@@ -97,7 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // We know these standard facets are always installed in every locale
       // so dynamic_cast always succeeds, just use static_cast instead.
 #define _GLIBCXX_STD_FACET(...) \
-      if _GLIBCXX17_CONSTEXPR (__is_same(_Facet, __VA_ARGS__)) \
+      if _GLIBCXX_CONSTEXPR (__is_same(_Facet, __VA_ARGS__)) \
        return static_cast<const _Facet*>(__facets[__i])
 
       _GLIBCXX_STD_FACET(ctype<char>);
@@ -145,6 +147,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return static_cast<const _Facet*>(__facets[__i]);
 #endif
     }
+#pragma GCC diagnostic pop
 
   /**
    *  @brief  Test for the presence of a facet.
-- 
2.43.0