Make pointer_traits SFINAE friendly

include/boost/core/pointer_traits.hpp

@@ -1,5 +1,5 @@
 /*
-Copyright 2017-2018 Glen Joseph Fernandes
+Copyright 2017-2021 Glen Joseph Fernandes
 (glenjofe@gmail.com)
 
 Distributed under the Boost Software License, Version 1.0.
@@ -9,43 +9,23 @@ Distributed under the Boost Software License, Version 1.0.
 #define BOOST_CORE_POINTER_TRAITS_HPP
 
 #include <boost/config.hpp>
-#if !defined(BOOST_NO_CXX11_POINTER_TRAITS)
-#include <memory>
-#else
 #include <boost/core/addressof.hpp>
 #include <cstddef>
-#endif
 
 namespace boost {
-
-#if !defined(BOOST_NO_CXX11_POINTER_TRAITS)
-template<class T>
-struct pointer_traits
-    : std::pointer_traits<T> {
-    template<class U>
-    struct rebind_to {
-        typedef typename std::pointer_traits<T>::template rebind<U> type;
-    };
-};
-
-template<class T>
-struct pointer_traits<T*>
-    : std::pointer_traits<T*> {
-    template<class U>
-    struct rebind_to {
-        typedef U* type;
-    };
-};
-#else
 namespace detail {
 
+struct ptr_none { };
+
 template<class>
-struct ptr_void {
+struct ptr_valid {
     typedef void type;
 };
 
-template<class T>
-struct ptr_first;
+template<class>
+struct ptr_first {
+    typedef ptr_none type;
+};
 
 #if !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)
 template<template<class, class...> class T, class U, class... Args>
@@ -75,7 +55,7 @@ struct ptr_element {
 };
 
 template<class T>
-struct ptr_element<T, typename ptr_void<typename T::element_type>::type> {
+struct ptr_element<T, typename ptr_valid<typename T::element_type>::type> {
     typedef typename T::element_type type;
 };
 
@@ -86,12 +66,12 @@ struct ptr_difference {
 
 template<class T>
 struct ptr_difference<T,
-    typename ptr_void<typename T::difference_type>::type> {
+    typename ptr_valid<typename T::difference_type>::type> {
     typedef typename T::difference_type type;
 };
 
-template<class T, class V>
-struct ptr_transform;
+template<class, class>
+struct ptr_transform { };
 
 #if !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)
 template<template<class, class...> class T, class U, class... Args, class V>
@@ -117,68 +97,100 @@ struct ptr_transform<T<U1, U2, U3>, V> {
 #endif
 
 template<class T, class U, class = void>
-struct ptr_rebind {
-    typedef typename ptr_transform<T, U>::type type;
-};
+struct ptr_rebind
+    : ptr_transform<T, U> { };
 
-#if !defined(BOOST_NO_CXX11_TEMPLATE_ALIASES)
 template<class T, class U>
 struct ptr_rebind<T, U,
-    typename ptr_void<typename T::template rebind<U> >::type> {
+    typename ptr_valid<typename T::template rebind<U> >::type> {
     typedef typename T::template rebind<U> type;
 };
-#endif
 
-template<class T>
-struct ptr_value {
-    typedef T type;
+template<class>
+struct ptr_void {
+    BOOST_STATIC_CONSTEXPR bool value = false;
 };
 
 template<>
-struct ptr_value<void> {
-    typedef struct { } type;
+struct ptr_void<void> {
+    BOOST_STATIC_CONSTEXPR bool value = true;
 };
 
+template<>
+struct ptr_void<const void> {
+    BOOST_STATIC_CONSTEXPR bool value = true;
+};
+
+template<>
+struct ptr_void<volatile void> {
+    BOOST_STATIC_CONSTEXPR bool value = true;
+};
+
+template<>
+struct ptr_void<const volatile void> {
+    BOOST_STATIC_CONSTEXPR bool value = true;
+};
+
+template<class, class E, bool = ptr_void<E>::value>
+struct ptr_to { };
+
+template<class T, class E>
+struct ptr_to<T, E, false> {
+    static T pointer_to(E& v) {
+        return T::pointer_to(v);
+    }
+};
+
+template<class T>
+struct ptr_to<T*, T, false> {
+    static T* pointer_to(T& v) BOOST_NOEXCEPT {
+        return boost::addressof(v);
+    }
+};
+
+template<class T, class E>
+struct ptr_traits
+    : ptr_to<T, E> {
+    typedef T pointer;
+    typedef E element_type;
+    typedef typename ptr_difference<T>::type difference_type;
+
+    template<class U>
+    struct rebind_to
+        : ptr_rebind<T, U> { };
+
+#if !defined(BOOST_NO_CXX11_TEMPLATE_ALIASES)
+    template<class U>
+    using rebind = typename rebind_to<U>::type;
+#endif
+};
+
+template<class T>
+struct ptr_traits<T, ptr_none> { };
+
 } /* detail */
 
 template<class T>
-struct pointer_traits {
-    typedef T pointer;
-    typedef typename detail::ptr_element<T>::type element_type;
-    typedef typename detail::ptr_difference<T>::type difference_type;
-    template<class U>
-    struct rebind_to {
-        typedef typename detail::ptr_rebind<T, U>::type type;
-    };
-#if !defined(BOOST_NO_CXX11_TEMPLATE_ALIASES)
-    template<class U>
-    using rebind = typename detail::ptr_rebind<T, U>::type;
-#endif
-    static pointer
-    pointer_to(typename detail::ptr_value<element_type>::type& v) {
-        return pointer::pointer_to(v);
-    }
-};
+struct pointer_traits
+    : detail::ptr_traits<T, typename detail::ptr_element<T>::type> { };
 
 template<class T>
-struct pointer_traits<T*> {
+struct pointer_traits<T*>
+    : detail::ptr_to<T*, T> {
     typedef T* pointer;
     typedef T element_type;
     typedef std::ptrdiff_t difference_type;
+
     template<class U>
     struct rebind_to {
         typedef U* type;
     };
+
 #if !defined(BOOST_NO_CXX11_TEMPLATE_ALIASES)
     template<class U>
-    using rebind = U*;
+    using rebind = typename rebind_to<U>::type*;
 #endif
-    static T*
-    pointer_to(typename detail::ptr_value<T>::type& v) BOOST_NOEXCEPT {
-        return boost::addressof(v);
-    }
 };
-#endif
 
 template<class T>
 BOOST_CONSTEXPR inline T*