warn if $set is used on a property that already exist

Version

2.5.16

Reproduction link

https://codepen.io/JJPandari/pen/gzLVBq?editors=1010

Steps to reproduce

See the codepen snippet. Follow the comment there to change the vm's data and see what happens.

What is expected?

Even if the prop already exists, using set still makes it reactive, thus trigger view update.

What is actually happening?

Using set later doesn't update the view.

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Related source code: https://github.com/vuejs/vue/blob/3eb37acf98e2d9737de897ebe7bdb7e9576bcc21/src/core/observer/index.js#L192 I think most users would expect set to make the prop reactive whenever it's used. I initially opened an issue for the api doc because it wasn't clear (for me) about this. But the comment in the source code is.