cgen: fix vweb using generic method (fix #15888) (#15963)

vlib/vweb/vweb_app_test.v

@@ -64,3 +64,31 @@ fn (mut app App) time_json_pretty() {
 		'time': time.now().format()
 	})
 }
+
+struct ApiSuccessResponse<T> {
+	success bool
+	result  T
+}
+
+fn (mut app App) json_success<T>(result T) vweb.Result {
+	response := ApiSuccessResponse<T>{
+		success: true
+		result: result
+	}
+
+	return app.json(response)
+}
+
+// should compile, this is a helper method, not exposed as a route
+fn (mut app App) some_helper<T>(result T) ApiSuccessResponse<T> {
+	response := ApiSuccessResponse<T>{
+		success: true
+		result: result
+	}
+	return response
+}
+
+// should compile, the route method itself is not generic
+fn (mut app App) ok() vweb.Result {
+	return app.json(app.some_helper(123))
+}