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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/113099] locale without RTTI uses dynamic_cast before
 gcc 13.2 or has ODR violation since gcc 13.2
Date: Sun, 24 Dec 2023 14:44:44 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 11.4.0
X-Bugzilla-Keywords: ABI, documentation
X-Bugzilla-Severity: normal
X-Bugzilla-Who: redi at gcc dot gnu.org
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Message-ID: <bug-113099-4-7Jx9vY1CcZ@http.gcc.gnu.org/bugzilla/>
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113099

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I'm not sure what you mean by "the compiler is free to generate code that t=
akes
it into account." Takes what into account? What problem does that freedom
cause?

The locale facet instantiations are compiled as C++11 (but other parts of t=
he
library are compiled as C++17 or newer). We should do this so it uses
if-constexpr for everything except C++98:

--- 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.=