decoder_detail: static_assert member function is from visitor class

Improves readability of compiler errors.

src/frontend/decoder/decoder_detail.h

@@ -27,6 +27,7 @@ template<class MatcherT>
 struct detail {
 private:
     using opcode_type  = typename MatcherT::opcode_type;
+    using visitor_type = typename MatcherT::visitor_type;
 
     static constexpr size_t opcode_bitsize = Common::BitSize<opcode_type>();
 
@@ -113,6 +114,7 @@ private:
                          CallRetT (Visitor::* const fn)(Args...),
                          const std::array<opcode_type, sizeof...(iota)> arg_masks,
                          const std::array<size_t, sizeof...(iota)> arg_shifts) {
+            static_assert(std::is_same<visitor_type, Visitor>::value, "Member function is not from Matcher's Visitor");
             return [fn, arg_masks, arg_shifts](Visitor& v, opcode_type instruction) {
                 (void)instruction;
                 return (v.*fn)(static_cast<Args>((instruction & arg_masks[iota]) >> arg_shifts[iota])...);

src/frontend/decoder/matcher.h

@@ -27,6 +27,7 @@ template <typename Visitor, typename OpcodeType>
 class Matcher {
 public:
     using opcode_type         = OpcodeType;
+    using visitor_type        = Visitor;
     using handler_return_type = typename Visitor::instruction_return_type;
     using handler_function    = std::function<handler_return_type(Visitor&, opcode_type)>;