libstdc++: Overload std::__to_address for __gnu_cxx::__normal_iterator.

Prefer to overload __to_address to partially specialize std::pointer_traits because
std::pointer_traits would be mostly useless. Moreover partial specialization of
pointer_traits<__normal_iterator<P, C>> fails to rebind C, so you get incorrect types
like __normal_iterator<long*, vector<int>>. In the case of __gnu_debug::_Safe_iterator
the to_pointer method is impossible to implement correctly because we are missing
the parent container to associate the iterator to.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h
	(std::pointer_traits<__gnu_cxx::__normal_iterator<>>): Remove.
	(std::__to_address(const __gnu_cxx::__normal_iterator<>&)): New for C++11 to C++17.
	* include/debug/safe_iterator.h
	(std::__to_address(const __gnu_debug::_Safe_iterator<__gnu_cxx::__normal_iterator<>,
	_Sequence>&)): New for C++11 to C++17.
	* testsuite/24_iterators/normal_iterator/to_address.cc: Add check on std::vector::iterator
	to validate both __gnu_cxx::__normal_iterator<> __to_address overload in normal mode and
	__gnu_debug::_Safe_iterator in _GLIBCXX_DEBUG mode.

libstdc++-v3/include/bits/stl_iterator.h

@@ -1349,32 +1349,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #if __cplusplus >= 201103L
 
-  // Need to specialize pointer_traits because the primary template will
-  // deduce element_type of __normal_iterator<T*, C> as T* rather than T.
+#if __cplusplus <= 201703L
+  // Need to overload __to_address because the pointer_traits primary template
+  // will deduce element_type of __normal_iterator<T*, C> as T* rather than T.
   template<typename _Iterator, typename _Container>
-    struct pointer_traits<__gnu_cxx::__normal_iterator<_Iterator, _Container>>
-    {
-    private:
-      using _Base = pointer_traits<_Iterator>;
-
-    public:
-      using element_type = typename _Base::element_type;
-      using pointer = __gnu_cxx::__normal_iterator<_Iterator, _Container>;
-      using difference_type = typename _Base::difference_type;
-
-      template<typename _Tp>
-	using rebind = __gnu_cxx::__normal_iterator<_Tp, _Container>;
-
-      static pointer
-      pointer_to(element_type& __e) noexcept
-      { return pointer(_Base::pointer_to(__e)); }
-
-#if __cplusplus >= 202002L
-      static element_type*
-      to_address(pointer __p) noexcept
-      { return __p.base(); }
+    constexpr auto
+    __to_address(const __gnu_cxx::__normal_iterator<_Iterator,
+						    _Container>& __it) noexcept
+    -> decltype(std::__to_address(__it.base()))
+    { return std::__to_address(__it.base()); }
 #endif
-    };
 
   /**
    * @addtogroup iterators

libstdc++-v3/include/debug/safe_iterator.h

@@ -1013,6 +1013,23 @@ namespace __gnu_debug
 
 } // namespace __gnu_debug
 
+#if __cplusplus >= 201103L && __cplusplus <= 201703L
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+  template<typename _Iterator, typename _Container, typename _Sequence>
+    constexpr auto
+    __to_address(const __gnu_debug::_Safe_iterator<
+		 __gnu_cxx::__normal_iterator<_Iterator, _Container>,
+		 _Sequence>& __it) noexcept
+    -> decltype(std::__to_address(__it.base().base()))
+    { return std::__to_address(__it.base().base()); }
+
+_GLIBCXX_END_NAMESPACE_VERSION
+}
+#endif
+
 #undef _GLIBCXX_DEBUG_VERIFY_DIST_OPERANDS
 #undef _GLIBCXX_DEBUG_VERIFY_REL_OPERANDS
 #undef _GLIBCXX_DEBUG_VERIFY_EQ_OPERANDS

libstdc++-v3/testsuite/24_iterators/normal_iterator/to_address.cc

@@ -1,6 +1,19 @@
 // { dg-do compile { target { c++11 } } }
 #include <string>
+#include <vector>
 #include <memory>
 
-char* p = std::__to_address(std::string("1").begin());
-const char* q = std::__to_address(std::string("2").cbegin());
+#include <testsuite_allocator.h>
+
+char* p __attribute__((unused))
+  = std::__to_address(std::string("1").begin());
+const char* q __attribute__((unused))
+  = std::__to_address(std::string("2").cbegin());
+int* r __attribute__((unused))
+  = std::__to_address(std::vector<int>(1, 1).begin());
+const int* s __attribute__((unused))
+  = std::__to_address(std::vector<int>(1, 1).cbegin());
+int* t __attribute__((unused))
+  = std::__to_address(std::vector<int, __gnu_test::CustomPointerAlloc<int>>(1, 1).begin());
+const int* u __attribute__((unused))
+  = std::__to_address(std::vector<int, __gnu_test::CustomPointerAlloc<int>>(1, 1).cbegin());