Make pointer_to SFINAE friendlier

include/boost/core/pointer_traits.hpp

@@ -106,43 +106,74 @@ struct ptr_rebind<T, U,
     typedef typename T::template rebind<U> type;
 };
 
-template<class>
+#if !defined(BOOST_NO_CXX11_DECLTYPE_N3276)
-struct ptr_void {
+template<class T, class E>
+class ptr_to_expr {
+    template<class>
+    struct result {
+        char x, y;
+    };
+
+    static E& source();
+
+    template<class O>
+    static auto check(int) -> result<decltype(O::pointer_to(source()))>;
+
+    template<class>
+    static char check(long);
+
+public:
+    BOOST_STATIC_CONSTEXPR bool value = sizeof(check<T>(0)) != 1;
+};
+
+template<class T, class E>
+struct ptr_to_expr<T*, E> {
+    BOOST_STATIC_CONSTEXPR bool value = true;
+};
+
+template<class T, class E>
+struct ptr_has_to {
+    BOOST_STATIC_CONSTEXPR bool value = ptr_to_expr<T, E>::value;
+};
+#else
+template<class, class>
+struct ptr_has_to {
+    BOOST_STATIC_CONSTEXPR bool value = true;
+};
+#endif
+
+template<class T>
+struct ptr_has_to<T, void> {
     BOOST_STATIC_CONSTEXPR bool value = false;
 };
 
-template<>
+template<class T>
-struct ptr_void<void> {
+struct ptr_has_to<T, const void> {
-    BOOST_STATIC_CONSTEXPR bool value = true;
+    BOOST_STATIC_CONSTEXPR bool value = false;
 };
 
-template<>
+template<class T>
-struct ptr_void<const void> {
+struct ptr_has_to<T, volatile void> {
-    BOOST_STATIC_CONSTEXPR bool value = true;
+    BOOST_STATIC_CONSTEXPR bool value = false;
 };
 
-template<>
+template<class T>
-struct ptr_void<volatile void> {
+struct ptr_has_to<T, const volatile void> {
-    BOOST_STATIC_CONSTEXPR bool value = true;
+    BOOST_STATIC_CONSTEXPR bool value = false;
 };
 
-template<>
+template<class T, class E, bool = ptr_has_to<T, E>::value>
-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> {
+struct ptr_to<T, E, true> {
     static T pointer_to(E& v) {
         return T::pointer_to(v);
     }
 };
 
 template<class T>
-struct ptr_to<T*, T, false> {
+struct ptr_to<T*, T, true> {
     static T* pointer_to(T& v) BOOST_NOEXCEPT {
         return boost::addressof(v);
     }

test/pointer_traits_pointer_to_sfinae_test.cpp

@@ -6,17 +6,15 @@ Distributed under the Boost Software License, Version 1.0.
 (http://www.boost.org/LICENSE_1_0.txt)
 */
 #include <boost/config.hpp>
-#if !defined(BOOST_NO_CXX11_DECLTYPE_N3276) || !defined(BOOST_MSVC)
+#if !defined(BOOST_NO_CXX11_DECLTYPE_N3276)
 #include <boost/core/pointer_traits.hpp>
 #include <boost/core/lightweight_test.hpp>
 
-#if !defined(BOOST_NO_CXX11_DECLTYPE_N3276)
 template<class T>
 class has_pointer_to {
     template<class>
     struct result {
-        char one;
+        char x, y;
-        char two;
     };
 
     template<class O>
@@ -28,41 +26,40 @@ class has_pointer_to {
 public:
     static const bool value = sizeof(check<T>(0)) != 1;
 };
-#else
-template<class T>
-class has_pointer_to {
-    template<int>
-    struct result {
-        char one;
-        char two;
-    };
-
-    template<class O>
-    static result<sizeof(&O::pointer_to)> check(int);
-
-    template<class>
-    static char check(long);
-
-public:
-    static const bool value = sizeof(check<T>(0)) != 1;
-};
-#endif
 
 struct P1 { };
 
 struct P2 {
     typedef int element_type;
+
+    static int* pointer_to(int& value) {
+        return &value;
+    }
 };
 
 struct P3 {
     typedef void element_type;
 };
 
+struct P4 {
+    typedef int element_type;
+};
+
+struct P5 {
+    typedef int element_type;
+
+    static int* pointer_to() {
+        return 0;
+    }
+};
+
 int main()
 {
     BOOST_TEST((!has_pointer_to<boost::pointer_traits<P1> >::value));
     BOOST_TEST(has_pointer_to<boost::pointer_traits<P2> >::value);
     BOOST_TEST(!has_pointer_to<boost::pointer_traits<P3> >::value);
+    BOOST_TEST(!has_pointer_to<boost::pointer_traits<P4> >::value);
+    BOOST_TEST(!has_pointer_to<boost::pointer_traits<P5> >::value);
     BOOST_TEST(has_pointer_to<boost::pointer_traits<int*> >::value);
     BOOST_TEST(!has_pointer_to<boost::pointer_traits<void*> >::value);
     return boost::report_errors();