fix(mergeData): skip recursive call if values are identical

[KaelWD]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

Other information:
Fixes vuetifyjs/vuetify#5633

I've been unable to reproduce this in isolation, but have run into it several times in the past. This check should also reduce unnecessary calls to mergeData, I noticed thousands of instances of identical objects being merged that didn't cause errors.

[posva]

I don't know what scenario causes this, but shouldn't be possible to reproduce with

const a = { a: 'a' }
const b = { a }
const c = { a }
// making the code call mergeData(b, c) somewhere

[KaelWD]

Yeah, but I don't know where to put b and c to make that happen. It only causes a stack overflow if a has recursive references, in our case it's a provided vue instance. In your example it would just be an unnecessary loop over a that doesn't do anything as all the properties obviously exist in both arguments. You can see some more information in the linked issue https://github.com/vuetifyjs/vuetifyjs.com/issues/630

[posva]

It's worth checking what the actual source of the problem is. There could be another problem somewhere else. That being said, the change is a good idea IMO

[KaelWD]

There's a similar issue #6190, but that's trying to merge two different vue objects so I don't think this patch will fix it unless it's getting hung up on $root or something.

John thinks this one has something do do with component reuse, because it only happens when you leave the page for another one. But yeah this does fix our particular problem, and probably has a small performance boost too so there's no reason not to include it.

[jvbianchi]

Any plans on merging this?

[jrvaja]

@KaelWD Hope this will be merged in the up coming sprint.

[whoisarpit]

Hi!
Wondering if the actual source of the problem was ever discovered?