libstdc++: Specialize std::pointer_traits<__normal_iterator<I,C>>

This allows std::__to_address to be used with __normal_iterator in
C++11/14/17 modes. Without the partial specialization the deduced
pointer_traits::element_type is incorrect, and so the return type of
__to_address is wrong.

A similar partial specialization is probably needed for
__gnu_debug::_Safe_iterator.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (pointer_traits): Define partial
	specialization for __normal_iterator.
	* testsuite/24_iterators/normal_iterator/to_address.cc: New test.

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

@@ -1285,6 +1285,34 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __it.base(); }
 
 #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.
+  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(); }
+#endif
+    };
+
   /**
    * @addtogroup iterators
    * @{

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

@@ -0,0 +1,6 @@
+// { dg-do compile { target { c++11 } } }
+#include <string>
+#include <memory>
+
+char* p = std::__to_address(std::string("1").begin());
+const char* q = std::__to_address(std::string("2").cbegin());