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

[EricWF]

Summary

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

[• rodrigc]

I'm part of the https://jenkins.freebsd.org continuous integration team,
and have been working with Dimitry Andric to test compiing FreeBSD-CURRENT
with gcc 4.9. I reported the original problem which Dimitry filed under PR22771.

I took your patch, and I can confirm that with it, I can compile libc++ with gcc 4.9

[dim]

This revision fixes both the original std::pair based test case I submitted in PR22771, and Richard Smith's minimal std::is_convertible based test case. Thanks. :)

[dim]

Note: Richard's minimal test case still does not compile with gcc 4.7.4, but that may very well be a gcc 4.7.4 bug:

$ gcc47 -I /share/dim/src/libcxx/trunk/include -std=c++11 -c minimal.cpp
/share/dim/src/libcxx/trunk/include/type_traits: In instantiation of 'struct std::__1::__is_convertible<const X&, X, 0u, 0u>':
/share/dim/src/libcxx/trunk/include/type_traits:957:62:   required from 'struct std::__1::is_convertible<const X&, X>'
minimal.cpp:4:42:   required from here
minimal.cpp:3:11: error: 'X::X(const X&)' is private
In file included from minimal.cpp:2:0:
/share/dim/src/libcxx/trunk/include/type_traits:893:8: error: within this context
/share/dim/src/libcxx/trunk/include/type_traits: In instantiation of 'struct std::__1::__is_convertible_imp::__is_convertible_test<const X&, X, void>':
/share/dim/src/libcxx/trunk/include/type_traits:893:8:   required from 'struct std::__1::__is_convertible<const X&, X, 0u, 0u>'
/share/dim/src/libcxx/trunk/include/type_traits:957:62:   required from 'struct std::__1::is_convertible<const X&, X>'
minimal.cpp:4:42:   required from here
minimal.cpp:3:11: error: 'X::X(const X&)' is private
In file included from minimal.cpp:2:0:
/share/dim/src/libcxx/trunk/include/type_traits:859:1: error: within this context
/share/dim/src/libcxx/trunk/include/type_traits:851:28: error:   initializing argument 1 of 'void std::__1::__is_convertible_imp::__test_convert(_Tp) [with _Tp = X]'
minimal.cpp:3:11: error: 'X::X(const X&)' is private
In file included from minimal.cpp:2:0:
/share/dim/src/libcxx/trunk/include/type_traits:859:1: error: within this context
/share/dim/src/libcxx/trunk/include/type_traits:851:28: error:   initializing argument 1 of 'void std::__1::__is_convertible_imp::__test_convert(_Tp) [with _Tp = X]'

The other versions I tested with had no problems:

• gcc48 (FreeBSD Ports Collection) 4.8.5 20150319 (prerelease)
• gcc49 (FreeBSD Ports Collection) 4.9.3 20150318 (prerelease)
• gcc5 (FreeBSD Ports Collection) 5.0.0 20150322 (experimental)

[EricWF]

Thanks for the thorough review. I suspect that 4.7 doesn't yes support access control SFINAE.

include/type_traits

 
 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();

     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)

test/std/utilities/meta/meta.rel/is_convertible.pass.cpp

     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
+
 }