[PATCH] [libcxx] Fix PR22771 - Support access control SFINAE in the library version of is_convertible.

[Eric Fiselier]

Hi mclow.lists, rsmith,

Currently the conversion check does not take place in a context where access control SFINAE is applied. This patch changes the context of the test expression so that SFINAE occurs if access control does not permit the conversion.

Related bug: https://llvm.org/bugs/show_bug.cgi?id=22771

http://reviews.llvm.org/D8461

Files:
  include/type_traits
  test/std/utilities/meta/meta.rel/is_convertible.pass.cpp

Index: include/type_traits
===================================================================
--- include/type_traits
+++ include/type_traits
@@ -848,7 +848,16 @@
 
 namespace __is_convertible_imp
 {
-template <class _Tp> char  __test(_Tp);
+template <class _Tp> void  __test_convert(_Tp);
+
+template <class _From, class _To, class = void>
+struct __is_convertible_test : public false_type {};
+
+template <class _From, class _To>
+struct __is_convertible_test<_From, _To,
+    decltype(__test_convert<_To>(_VSTD::declval<_From>()))> : public true_type
+{};
+
 template <class _Tp> __two __test(...);
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
 template <class _Tp> _Tp&& __source();
@@ -883,10 +892,8 @@
     unsigned _T2_is_array_function_or_void = __is_convertible_imp::__is_array_function_or_void<_T2>::value>
 struct __is_convertible
     : public integral_constant<bool,
-#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
-        sizeof(__is_convertible_imp::__test<_T2>(__is_convertible_imp::__source<_T1>())) == 1
-#else
-        sizeof(__is_convertible_imp::__test<_T2>(__is_convertible_imp::__source<_T1>())) == 1
+        __is_convertible_imp::__is_convertible_test<_T1, _T2>::value
+#if defined(_LIBCPP_HAS_NO_RVALUE_REFERENCES)
          && !(!is_function<_T1>::value && !is_reference<_T1>::value && is_reference<_T2>::value
               && (!is_const<typename remove_reference<_T2>::type>::value
                   || is_volatile<typename remove_reference<_T2>::type>::value)
Index: test/std/utilities/meta/meta.rel/is_convertible.pass.cpp
===================================================================
--- test/std/utilities/meta/meta.rel/is_convertible.pass.cpp
+++ test/std/utilities/meta/meta.rel/is_convertible.pass.cpp
@@ -186,4 +186,10 @@
     static_assert((std::is_convertible<volatile NonCopyable&, const volatile NonCopyable&>::value), "");
     static_assert((std::is_convertible<const volatile NonCopyable&, const volatile NonCopyable&>::value), "");
     static_assert((!std::is_convertible<const NonCopyable&, NonCopyable&>::value), "");
+// This test requires Access control SFINAE which we only have in C++11 or when
+// we are using the compiler builtin for is_convertible.
+#if __cplusplus >= 201103L || !defined(_LIBCPP_USE_IS_CONVERTIBLE_FALLBACK)
+    test_is_not_convertible<NonCopyable&, NonCopyable>();
+#endif
+
 }

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