### Version 2.6.0 ### Reproduction link [https://codesandbox.io/embed/vue-template-hf9hq](https://codesandbox.io/embed/vue-template-hf9hq) ### Steps to reproduce - Click on 'route' link - Look at the console ### What is expected? In `components/Parent.vue` the object `childRoute`, that is passed as the `to` prop of `<router-link>`, should not be mutated. ### What is actually happening? It is mutated (the key `parent` is added to `childRoute.params`) <!-- generated by vue-issues. DO NOT REMOVE -->