[gcc r13-2884] libstdc++: Update std::pointer_traits to match new LWG 3545 wording

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:03cb9ed8dd603dbb77762ca948fc6381ba190731

commit r13-2884-g03cb9ed8dd603dbb77762ca948fc6381ba190731
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Sep 26 18:59:45 2022 +0100

    libstdc++: Update std::pointer_traits to match new LWG 3545 wording
    
    It was pointed out in recent LWG 3545 discussion that having a
    constrained partial specialization of std::pointer_traits can cause
    ambiguities with program-defined specializations. For example, the
    addition to the testcase has:
    
    template<typename P> requires std::derived_from<P, base_type
    struct std::pointer_traits<P>;
    
    This would be ambiguous with the library's own constrained partial
    specialization:
    
    template<typename Ptr> requires requires { typename Ptr::element_type; }
    struct std::pointer_traits<Ptr>;
    
    Neither specialization is more specialized than the other for a type
    that is derived from base_type and also has an element_type member.
    
    The solution is to remove the library's partial specialization, and do
    the check for Ptr::element_type in the __ptr_traits_elem helper (which
    is what we already do for !__cpp_concepts anyway).
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/ptr_traits.h (__ptr_traits_elem) [__cpp_concepts]:
            Also define the __ptr_traits_elem class template for the
            concepts case.
            (pointer_traits<Ptr>): Remove constrained partial
            specialization.
            * testsuite/20_util/pointer_traits/lwg3545.cc: Check for
            ambiguitiy with program-defined partial specialization.

Diff:
---
 libstdc++-v3/include/bits/ptr_traits.h               | 20 ++++++--------------
 .../testsuite/20_util/pointer_traits/lwg3545.cc      | 17 +++++++++++++++++
 2 files changed, 23 insertions(+), 14 deletions(-)

diff --git a/libstdc++-v3/include/bits/ptr_traits.h b/libstdc++-v3/include/bits/ptr_traits.h
index ae8810706ab..71370ff4fc9 100644
--- a/libstdc++-v3/include/bits/ptr_traits.h
+++ b/libstdc++-v3/include/bits/ptr_traits.h
@@ -73,25 +73,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct __replace_first_arg<_SomeTemplate<_Tp, _Types...>, _Up>
     { using type = _SomeTemplate<_Up, _Types...>; };
 
-#if __cpp_concepts
-  // When concepts are supported detection of _Ptr::element_type is done
-  // by a requires-clause, so __ptr_traits_elem_t only needs to do this:
-  template<typename _Ptr>
-    using __ptr_traits_elem_t = typename __get_first_arg<_Ptr>::type;
-#else
   // Detect the element type of a pointer-like type.
   template<typename _Ptr, typename = void>
     struct __ptr_traits_elem : __get_first_arg<_Ptr>
     { };
 
   // Use _Ptr::element_type if is a valid type.
+#if __cpp_concepts
+  template<typename _Ptr> requires requires { typename _Ptr::element_type; }
+    struct __ptr_traits_elem<_Ptr, void>
+    { using type = typename _Ptr::element_type; };
+#else
   template<typename _Ptr>
     struct __ptr_traits_elem<_Ptr, __void_t<typename _Ptr::element_type>>
     { using type = typename _Ptr::element_type; };
+#endif
 
   template<typename _Ptr>
     using __ptr_traits_elem_t = typename __ptr_traits_elem<_Ptr>::type;
-#endif
 
   /// @endcond
 
@@ -182,13 +181,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct pointer_traits : __ptr_traits_impl<_Ptr, __ptr_traits_elem_t<_Ptr>>
     { };
 
-#if __cpp_concepts
-  template<typename _Ptr> requires requires { typename _Ptr::element_type; }
-    struct pointer_traits<_Ptr>
-    : __ptr_traits_impl<_Ptr, typename _Ptr::element_type>
-    { };
-#endif
-
   /**
    * @brief  Partial specialization for built-in pointers.
    * @headerfile memory
diff --git a/libstdc++-v3/testsuite/20_util/pointer_traits/lwg3545.cc b/libstdc++-v3/testsuite/20_util/pointer_traits/lwg3545.cc
index 08c3ed01b75..93c64a353bd 100644
--- a/libstdc++-v3/testsuite/20_util/pointer_traits/lwg3545.cc
+++ b/libstdc++-v3/testsuite/20_util/pointer_traits/lwg3545.cc
@@ -99,3 +99,20 @@ static_assert( is_same<pointer<Ctraits>, clever_ptr<char>>::value, "" );
 static_assert( is_same<difference_type<Ctraits>, std::ptrdiff_t>::value, "" );
 static_assert( is_same<rebind<Ctraits>, clever_ptr<short>>::value, "" );
 static_assert( is_same<pointer_to<Ctraits>, clever_ptr<char>>::value, "" );
+
+#ifdef __cpp_concepts
+struct ptr_base { };
+
+// Program-defined specialization must not be ambiguous with primary template.
+template<typename P> requires std::derived_from<P, ptr_base>
+struct std::pointer_traits<P>
+{
+  using element_type = int;
+  using difference_type = long;
+  using pointer = P;
+};
+
+struct Ptr : ptr_base { using element_type = int; };
+
+using E = std::pointer_traits<Ptr>::element_type;
+#endif