Add support to accept allocator arguments with value_type == void.

example/doc_void_allocator.cpp

@@ -0,0 +1,72 @@
+//////////////////////////////////////////////////////////////////////////////
+//
+// (C) Copyright Ion Gaztanaga 2026. Distributed under the Boost
+// Software License, Version 1.0. (See accompanying file
+// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// See http://www.boost.org/libs/container for documentation.
+//
+//////////////////////////////////////////////////////////////////////////////
+
+#include <boost/container/detail/type_traits.hpp>
+
+//[doc_void_allocator
+#include <boost/container/vector.hpp>
+#include <boost/container/map.hpp>
+#include <boost/container/allocator.hpp>
+#include <boost/container/pmr/polymorphic_allocator.hpp>
+#include <utility>
+//=#include <type_traits>
+
+
+int main()
+{
+   using namespace boost::container;
+
+//
+// void as Allocator template argument
+//
+   //Allocator argument is void: the library selects its default allocator.
+   vector<int>       v_default;          // Allocator defaults to void
+   vector<int, void> v_explicit_void;    // Explicit void
+   map<int, double>  m_default;          // Allocator defaults to void
+   map<int, double, std::less<int>, void> m_explicit_void;
+
+   //Same types
+   v_default.push_back(1);
+   v_explicit_void = v_default;
+
+   m_default[3] = 3.0;
+   m_explicit_void = m_default;
+
+//
+// Automatic rebinding with allocator::value_type == void, 
+//
+   typedef pmr::polymorphic_allocator<void>              pmr_void_t;
+   typedef vector<int, pmr_void_t >                      vector_alloc_of_void_t;
+   typedef map<int, double, std::less<int>, pmr_void_t > map_alloc_of_void_t;
+   typedef std::pair<const int, double>                  map_value_t;
+
+   /*<-*/
+   BOOST_CONTAINER_STATIC_ASSERT
+      ((dtl::is_same< vector_alloc_of_void_t::allocator_type
+      , pmr::polymorphic_allocator<int> >::value));
+   BOOST_CONTAINER_STATIC_ASSERT
+      ((dtl::is_same< map_alloc_of_void_t::allocator_type
+      , pmr::polymorphic_allocator<map_value_t> >::value));
+   /*->*/
+   //Container::allocator_type is the expected type
+   //=static_assert
+   //=   (std::is_same< vector_alloc_of_void_t >::allocator_type, pmr::polymorphic_allocator<int> >::value);
+   //=static_assert
+   //=   (std::is_same< map_alloc_of_void_t >::allocator_type, pmr::polymorphic_allocator<map_value_t> >::value);
+
+   //Usually the Allocator<void> type is convertible to the rebound allocator, no need
+   //to explicitly rebind it.
+   pmr_void_t alloc;
+   vector_alloc_of_void_t v(alloc);
+   map_alloc_of_void_t m(std::less<int>(), alloc);
+
+   return 0;
+}
+//]