Private member access in template substitution and SFINAE

class A { int a; };

template<typename, typename = void>
class test {};

template<typename T>
class test<T,decltype(T::a)> {};

int main() { test<A> a; }

The code above compiles without error on clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/final), but fails to compile on g++-5 (Ubuntu 5.4.1-2ubuntu1~16.04) 5.4.1 20160904 and g++-6 (Ubuntu 6.2.0-3ubuntu11~16.04) 6.2.0 20160901 with errors like this:

main.cpp: In function ‘int main()’:
main.cpp:9:22: error: ‘int A::a’ is private within this context
 int main() { test<A> a; }
                      ^
main.cpp:1:15: note: declared private here
 class A { int a; };

In both cases I compiled with -std=c++11, but the effect is the same for -std=c++14 and -std=c++1z.

Which compiler is correct here? I assumed that, at least since C++11, accessing private members during template substitution should trigger SFINAE, implying that clang is correct and gcc is not. Is there some additional rule I am unaware of?

For referencce, I am thinking of the note in §14.8.2/8 of the current standard draft N4606:

If a substitution results in an invalid type or expression, type deduction fails. An invalid type or expression is one that would be ill-formed, with a diagnostic required, if written using the substituted arguments. [ Note: If no diagnostic is required, the program is still ill-formed. Access checking is done as part of the substitution process. — end note ]

Using a member function and a member function pointer instead is accepted by both compilers:

class A { void a() {}; };

template<typename, typename = void>
class test {};

template<typename T>
class test<T,decltype(&T::a)> {};

int main() { test<A> a; }