watch reactive array callback called with new value only

[randName]

Version

3.0.0-rc.6

Reproduction link

https://codesandbox.io/s/quiet-dream-jwnql?file=/src/index.js

Steps to reproduce

watch a reactive array, the callback function is called with the new value only

What is expected?

second argument should be the old value

What is actually happening?

both arguments are the new value

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potentially related to #1655 . Unsure if this is the proper way to watch for an array, since this returned a plain array before #1656 (it is a Proxy now)

Also is there a more idomatic way to watch for changes in an array of objects? e.g. leveraging what Vue already does for DOM updates.

[posva]

See the note at https://vuejs.org/v2/api/#vm-watch, this is expected when mutating objects. Will pass on the information about the note missing in v3.