gcc_sfinae_issue.cpp

#include <iostream>
#include <type_traits>

template<class T> class HasTypeTag {
	// This version doesn't work as intended in GCC
	template<class C> static std::true_type test(char[sizeof(C::TypeTag)]);
	// This version works in GCC, Clang and VS2013
	//template<class C> static auto test(void*) -> decltype(C::TypeTag, std::true_type());
	template<class C> static std::false_type test(...);
public:
	enum { Value = std::is_same<decltype(test<T>(nullptr)), std::true_type>::value };
};

struct Foo { enum { TypeTag = 42 }; };
struct Bar {};

int main() {
	static_assert(HasTypeTag<Foo>::Value == true, "Foo should have TypeTag");
	static_assert(HasTypeTag<Bar>::Value == false, "Bar shouldn't have TypeTag");
	std::cout << "Foo: " << HasTypeTag<Foo>::Value << std::endl;
	std::cout << "Bar: " << HasTypeTag<Bar>::Value << std::endl;
	return 0;
}