[gcc r16-7485] libstdc++: Fix fallback definitions of std::is_member_*_pointer

[Jonathan Wakely]

https://gcc.gnu.org/g:db072f75e5ade3b9f1f2ff76fcdfda0d3bf028d1

commit r16-7485-gdb072f75e5ade3b9f1f2ff76fcdfda0d3bf028d1
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Feb 12 18:52:12 2026 +0000

    libstdc++: Fix fallback definitions of std::is_member_*_pointer
    
    When the builtins aren't used we need a declaration of std::is_function
    for the fallback definitions of std::is_member_function_pointer and
    std::is_member_object_pointer.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/type_traits (is_function): Declare before first
            use.

Diff:
---
 libstdc++-v3/include/std/type_traits | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index d533dc0ef09e..ea700d1ed97d 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -674,6 +674,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     : public __bool_constant<__is_member_object_pointer(_Tp)>
     { };
 #else
+  template<typename _Tp>
+    struct is_function;
+
   template<typename>
     struct __is_member_object_pointer_helper
     : public false_type { };
@@ -696,6 +699,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     : public __bool_constant<__is_member_function_pointer(_Tp)>
     { };
 #else
+  template<typename _Tp>
+    struct is_function;
+
   template<typename>
     struct __is_member_function_pointer_helper
     : public false_type { };