Custom options merge order

[skirtles-code]

Version

3.0.0-rc.10

Reproduction link

https://jsfiddle.net/skirtle/gxou4bqd/1/

Steps to reproduce

The console logging shows the calls made to merge the custom option.

What is expected?

The final result, as seen in the DOM, should be my-child.

What is actually happening?

The global mixin is merged last, so it takes priority.

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The problem here is not just with global mixins. The whole merge order for custom options is incorrect.

Here is an equivalent Vue 2 example. It shows the correct order:

https://jsfiddle.net/skirtle/btqcnj3p/1/

There is a spurious merge in Vue 2 that probably shouldn't be there (the one that logs global-mixin to undefined) but it doesn't seem to do any harm. Apart from that, everything happens in the order is should.

Roughly speaking, the correct order of precedence is:

1. Global mixins (lowest priority)
2. extends
3. mixins
4. The config object itself (highest priority)

For extends and mixins this order should be applied recursively.

It isn't just a problem for custom properties in optionMergeStrategies. Removing the strategy initially seems to fix the problem as the result is my-child. However, if you also remove the custom: 'my-child' it should fall back to mixin, whereas it actually falls back to extends.