error on bad usertypes

sol/function_types_usertype.hpp

@@ -165,6 +165,22 @@ struct usertype_indexing_function : base_function {
         return prelude(L);
     }
 };
+
+struct fail_on_error : base_function {
+    int prelude(lua_State* L) {
+        const char* accessor = stack::get<const char*>(L, 1 - lua_gettop(L));
+        return luaL_error(L, "sol: attempt to index nil value \"%s\" on userdata (bad (mispelled?) key name or does not exist)", accessor);
+    }
+
+    virtual int operator()(lua_State* L) override {
+        return prelude(L);
+    }
+};
+
+inline fail_on_error& failure_on_error() {
+    static fail_on_error f;
+    return f;
+}
 } // function_detail
 } // sol
 

sol/usertype.hpp

@@ -435,11 +435,17 @@ private:
     void build_function_tables() {
         int variableend = 0;
         if (!indexwrapper.empty()) {
+            if (indexfunc == nullptr) {
+                indexfunc = &function_detail::failure_on_error();
+            }
             functions.push_back(std::make_unique<function_detail::usertype_indexing_function>("__index", indexfunc, std::move(indexwrapper)));
             metafunctiontable.push_back({ "__index", function_detail::usertype_call<N> });
             ++variableend;
         }
         if (!newindexwrapper.empty()) {
+            if (newindexfunc == nullptr) {
+                newindexfunc = &function_detail::failure_on_error();
+            }
             functions.push_back(std::make_unique<function_detail::usertype_indexing_function>("__newindex", newindexfunc, std::move(newindexwrapper)));
             metafunctiontable.push_back({ "__newindex", indexwrapper.empty() ? function_detail::usertype_call<N> : function_detail::usertype_call<N + 1> });
             ++variableend;