commit fb4d55ef61ca3191ec946d4d41e0e715f4cc4197
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu May 6 13:44:36 2021

    libstdc++: Simplify __normal_iterator converting constructor
    
    This uses C++11 features to simplify the definition of the
    __normal_iterator constructor that allows converting from iterator to
    const_iterator. The previous definition relied on _Container::pointer
    which is present in std::vector and std::basic_string, but is not
    actually part of the container requirements.
    
    Removing the use of _Container::pointer and defining it in terms of
    is_convertible allows __normal_iterator to be used with new container
    types which do not define a pointer member. Specifically, this will
    allow it to be used in std::basic_stacktrace.
    
    In theory this will enable some conversions which were not previously
    permitted, for example __normal_iterator<volatile T*, vector<T>> can
    now be converted to __normal_iterator<const volatile T*, vector<T>>.
    In practice this doesn't matter because the library never uses such
    types.  In any case, allowing those conversions is consistent with
    the corresponding constructors of std::reverse_iterator and
    std::move_iterator.
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/stl_iterator.h (__normal_iterator): Simplify
            converting constructor and do not require _Container::pointer.

diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index 8517652a173..df774eeb63f 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -1022,6 +1022,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       typedef std::iterator_traits<_Iterator>		__traits_type;
 
+#if __cplusplus >= 201103L
+      template<typename _Iter>
+	using __convertible_from
+	  = std::__enable_if_t<std::is_convertible<_Iter, _Iterator>::value>;
+#endif
+
     public:
       typedef _Iterator					iterator_type;
       typedef typename __traits_type::iterator_category iterator_category;
@@ -1042,12 +1048,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       : _M_current(__i) { }
 
       // Allow iterator to const_iterator conversion
+#if __cplusplus >= 201103L
+      template<typename _Iter, typename = __convertible_from<_Iter>>
+	_GLIBCXX20_CONSTEXPR
+	__normal_iterator(const __normal_iterator<_Iter, _Container>& __i)
+	noexcept
+#else
+      // N.B. _Container::pointer is not actually in container requirements,
+      // but is present in std::vector and std::basic_string.
       template<typename _Iter>
-        _GLIBCXX20_CONSTEXPR
         __normal_iterator(const __normal_iterator<_Iter,
 			  typename __enable_if<
-      	       (std::__are_same<_Iter, typename _Container::pointer>::__value),
-		      _Container>::__type>& __i) _GLIBCXX_NOEXCEPT
+	       (std::__are_same<_Iter, typename _Container::pointer>::__value),
+		      _Container>::__type>& __i)
+#endif
         : _M_current(__i.base()) { }
 
       // Forward iterator requirements