Assert that both the underlying container and dynamic_bitset provide at least bidirectional iterators

The assert uses C++20 concepts, so it is only performed in C++20 or
later.

Note that, while evaluating the concept for the second assert, MSVC 2022
complained a constructor of class reference was private, therefore we
added a friend declaration.

include/boost/dynamic_bitset/dynamic_bitset.hpp

@@ -24,6 +24,7 @@
 #include "boost/dynamic_bitset_fwd.hpp"
 #include "boost/limits.hpp"
 #include <iosfwd>
+#include <iterator>
 #include <string>
 #include <type_traits>
 #include <vector>
@@ -197,6 +198,7 @@ public:
     class reference
     {
         friend class dynamic_bitset< Block, AllocatorOrContainer >;
+        friend class bit_iterator< dynamic_bitset >;
 
         //!     The one and only non-copy ctor
         // -------------------------------------------------------------------
@@ -294,6 +296,11 @@ public:
     // -----------------------------------------------------------------------
     typedef std::reverse_iterator< const_iterator > const_reverse_iterator;
 
+#if __cplusplus >= 202002L
+    static_assert( std::bidirectional_iterator< typename buffer_type::iterator >, "AllocatorOrContainer doesn't provide at least BidirectionalIterators" );
+    static_assert( std::bidirectional_iterator< iterator > );
+#endif
+
     //!     Constructs a bitset of size zero.
     //!
     //!     \par Postconditions

include/boost/dynamic_bitset/impl/dynamic_bitset.ipp

@@ -25,7 +25,6 @@
 #include <algorithm>
 #include <climits>
 #include <istream>
-#include <iterator>
 #include <ostream>
 #include <stdexcept>
 #include <utility>