clarify/fix std::visit compile time check

include/Common.h

@@ -132,6 +132,10 @@ private:
     static inline Version mVersion { 3, 9, 2 };
 };
 
+/// Used to static_assert in std::visit
+template<class>
+inline constexpr bool AlwaysFalseV = false;
+
 void SplitString(std::string const& str, const char delim, std::vector<std::string>& out);
 std::string LowerString(std::string str);