[gcc r14-7256] libstdc++: Fix redefinition error in std::tuple [PR108822]

[Jonathan Wakely]

https://gcc.gnu.org/g:1e88a151f878e0a139bf55eb0cee2506fb903274

commit r14-7256-g1e88a151f878e0a139bf55eb0cee2506fb903274
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Jan 15 16:51:39 2024 +0000

    libstdc++: Fix redefinition error in std::tuple [PR108822]
    
    When using a compiler that doesn't define __cpp_conditional_explicit
    there's a redefinition error for tuple::__nothrow_assignable. This is
    because it's defined in different places for the pre-C++20 and C++20
    implementations, which are controled by different preprocessor
    conditions. For certain combinations of C++20 feature test macros it's
    possible for both __nothrow_assignable definitions to be in scope.
    
    Move the pre-C++20 __assignable and __nothrow_assignable definitions adjacent to
    their use, so that only one set of definitions is visible for any given
    set of feature test macros.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/108822
            * include/std/tuple (__assignable, __is_nothrow_assignable):
            Move pre-C++20 definitions adjacent to their use.

Diff:
---
 libstdc++-v3/include/std/tuple | 35 +++++++++++++++++++----------------
 1 file changed, 19 insertions(+), 16 deletions(-)

diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index 5f4a393b532..2f6e419ea5b 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -1237,20 +1237,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  _TCC<_Cond>::template __is_explicitly_constructible<_Args...>(),
 	  bool>;
 
-      template<typename... _UElements>
-	static constexpr
-	__enable_if_t<sizeof...(_UElements) == sizeof...(_Elements), bool>
-	__assignable()
-	{ return __and_<is_assignable<_Elements&, _UElements>...>::value; }
-
-      // Condition for noexcept-specifier of an assignment operator.
-      template<typename... _UElements>
-	static constexpr bool __nothrow_assignable()
-	{
-	  return
-	    __and_<is_nothrow_assignable<_Elements&, _UElements>...>::value;
-	}
-
       // Condition for noexcept-specifier of a constructor.
       template<typename... _UElements>
 	static constexpr bool __nothrow_constructible()
@@ -1685,7 +1671,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	operator=(_UTuple&& __u) const;
 #endif // C++23
 
-#else // ! concepts
+#else // ! (concepts && consteval)
+
+    private:
+      template<typename... _UElements>
+	static constexpr
+	__enable_if_t<sizeof...(_UElements) == sizeof...(_Elements), bool>
+	__assignable()
+	{ return __and_<is_assignable<_Elements&, _UElements>...>::value; }
+
+      // Condition for noexcept-specifier of an assignment operator.
+      template<typename... _UElements>
+	static constexpr bool __nothrow_assignable()
+	{
+	  return
+	    __and_<is_nothrow_assignable<_Elements&, _UElements>...>::value;
+	}
+
+    public:
 
       _GLIBCXX20_CONSTEXPR
       tuple&
@@ -1728,7 +1731,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  this->_M_assign(std::move(__in));
 	  return *this;
 	}
-#endif // concepts
+#endif // concepts && consteval
 
       // tuple swap
       _GLIBCXX20_CONSTEXPR