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.

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 ae88107..71370ff 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 08c3ed0..93c64a3 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