Fixes #150 ("Use std::contiguous_iterator_tag if available"), tested in MSVC and GCC.

doc/container.qbk

@@ -1341,6 +1341,7 @@ use [*Boost.Container]? There are several reasons for that:
 [section:release_notes_boost_1_77_00 Boost 1.77 Release]
 
 * Fixed bugs/issues:
+   * [@https://github.com/boostorg/container/issues/150    GitHub #150: ['"Use std::contiguous_iterator_tag if available"]].
    * [@https://github.com/boostorg/container/issues/184    GitHub #184: ['"Issues with custom exceptions implementation"]].
 
 [endsect]

include/boost/container/detail/std_fwd.hpp

@@ -53,6 +53,9 @@ struct allocator_arg_t;
 
 struct piecewise_construct_t;
 
+template <class Ptr>
+struct pointer_traits;
+
 BOOST_MOVE_STD_NS_END
 #include <boost/move/detail/std_ns_end.hpp>
 

include/boost/container/vector.hpp

@@ -82,7 +82,17 @@ class vec_iterator
 {
    public:
    typedef std::random_access_iterator_tag                                          iterator_category;
+   typedef std::contiguous_iterator_tag                                             iterator_concept;
    typedef typename boost::intrusive::pointer_traits<Pointer>::element_type         value_type;
+
+   //Defining element_type to make libstdc++'s std::pointer_traits well-formed leads to ambiguity
+   //due to LWG3446. So we need to specialize std::pointer_traits. See 
+   //https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96416 for details. /Many thanks to Jonathan Wakely
+   //for explaning the issue.
+   #ifndef BOOST_GNU_STDLIB
+   //Define element_
+   typedef typename boost::intrusive::pointer_traits<Pointer>::element_type         element_type;
+   #endif
    typedef typename boost::intrusive::pointer_traits<Pointer>::difference_type      difference_type;
    typedef typename dtl::if_c
       < IsConst
@@ -3378,6 +3388,20 @@ struct has_trivial_destructor_after_move<boost::container::vector<T, Allocator,
 
 }
 
+//See comments on vec_iterator::element_type to know why is this needed
+#ifdef BOOST_GNU_STDLIB
+
+BOOST_MOVE_STD_NS_BEG
+
+template <class Pointer, bool IsConst>
+struct pointer_traits< boost::container::vec_iterator<Pointer, IsConst> >
+   : public boost::intrusive::pointer_traits< boost::container::vec_iterator<Pointer, IsConst> >
+{};
+
+BOOST_MOVE_STD_NS_END
+
+#endif   //BOOST_GNU_STDLIB
+
 #endif   //#ifndef BOOST_CONTAINER_DOXYGEN_INVOKED
 
 #include <boost/container/detail/config_end.hpp>

test/vector_test.cpp

@@ -194,7 +194,25 @@ bool test_merge_empty_free()
    vector< int, check_dealloc_allocator<int> > empty;
    empty.merge(source.begin(), source.end());
 
-   return empty.get_stored_allocator().deallocate_called_without_allocate_;
+   return !empty.get_stored_allocator().deallocate_called_without_allocate_;
+}
+
+#ifdef __cpp_lib_span
+#include <span>
+#endif
+
+bool test_span_conversion()
+{
+   #ifdef __cpp_lib_span
+   {
+      boost::container::vector myVec{1, 2, 3, 4, 5};
+      std::span mySpan1{myVec};                                        // (1)
+      std::span mySpan2{myVec.data(), myVec.size()};                   // (2)
+      return mySpan1.size() == myVec.size() && mySpan1.size() == mySpan2.size();
+   }
+   #else
+   return true;
+   #endif
 }
 
 int main()
@@ -331,11 +349,16 @@ int main()
    }
 #endif
 
-   if (test_merge_empty_free()) {
+   if (!test_merge_empty_free()) {
       std::cerr << "Merge into empty vector test failed" << std::endl;
       return 1;
    }
 
+   if (!test_span_conversion()) {
+      std::cerr << "Span conversion failed" << std::endl;
+      return 1;
+   }
+
    ////////////////////////////////////
    //    has_trivial_destructor_after_move testing
    ////////////////////////////////////