Add a flag to require explicit method not allowed handler

mux.go

@@ -51,6 +51,9 @@ type Router struct {
 	// Configurable Handler to be used when the request method does not match the route.
 	MethodNotAllowedHandler http.Handler
 
+	// If true, only handle MethodNotAllowed if the router MethodNotAllowedHandler is explicitly specified
+	RequireExplicitMethodNotAllowedHandler bool
+
 	// Routes to be matched, in order.
 	routes []*Route
 
@@ -199,7 +202,7 @@ func (r *Router) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 		req = requestWithRoute(req, match.Route)
 	}
 
-	if handler == nil && match.MatchErr == ErrMethodMismatch {
+	if handler == nil && match.MatchErr == ErrMethodMismatch && !r.RequireExplicitMethodNotAllowedHandler {
 		handler = methodNotAllowedHandler()
 	}
 

mux_test.go

@@ -2733,6 +2733,23 @@ func TestMethodNotAllowed(t *testing.T) {
 	}
 }
 
+func TestRequireExplicitMethodNotAllowedHandler(t *testing.T) {
+	handler := func(w http.ResponseWriter, r *http.Request) { w.WriteHeader(http.StatusOK) }
+	router := NewRouter()
+	router.RequireExplicitMethodNotAllowedHandler = true
+	router.HandleFunc("/thing", handler).Methods(http.MethodGet)
+	router.HandleFunc("/something", handler).Methods(http.MethodGet)
+
+	w := NewRecorder()
+	req := newRequest(http.MethodPut, "/thing")
+
+	router.ServeHTTP(w, req)
+
+	if w.Code != http.StatusNotFound {
+		t.Fatalf("Expected status code 404 (got %d)", w.Code)
+	}
+}
+
 type customMethodNotAllowedHandler struct {
 	msg string
 }