[gcc r12-3935] libstdc++: Specialize std::pointer_traits<__normal_iterator<I, C>>

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

https://gcc.gnu.org/g:82626be2d633a9802a8b08727ef51c627e37fee5

commit r12-3935-g82626be2d633a9802a8b08727ef51c627e37fee5
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Sep 28 15:26:46 2021 +0100

    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.

Diff:
---
 libstdc++-v3/include/bits/stl_iterator.h           | 28 ++++++++++++++++++++++
 .../24_iterators/normal_iterator/to_address.cc     |  6 +++++
 2 files changed, 34 insertions(+)

diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index c5b02408c1c..004d767224d 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/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
    * @{
diff --git a/libstdc++-v3/testsuite/24_iterators/normal_iterator/to_address.cc b/libstdc++-v3/testsuite/24_iterators/normal_iterator/to_address.cc
new file mode 100644
index 00000000000..510d627435f
--- /dev/null
+++ b/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());