I'm not a pro in C++ but somehow I provided a solution while porting my MSVS 2015 C++ code to MinGW 4.9.2 to specialize std::hash class to support all enums. It there's any C++ compiler developers or C++ pro programmers here, can you explain why this specialization works, although it is Undefined Behavior according the C++ standard they say?
Link for full code with samples on Gist
#include <unordered_set>
#include <functional>
#if (defined __GNUC__) && (__GNUC__ < 6)
// …