Add warning for ambiguous usage of `slot-scope` and `v-for` on the same element

[hit12369]

Version

2.5.2

Reproduction link

https://jsfiddle.net/xrecft4f/

Steps to reproduce

parent1 is not rendering anything, while parent2 is rendering correctly.

What is expected?

v-for should work if the slot-scope is given on the element.

What is actually happening?

As visible in the fiddle, parent1 is not rendering anything. Right now it only works if it's wrapped inside a template tag.

[HerringtonDarkholme]

I don't know if this should be fixed. I would suggest users to explicitly use template.
Compiler will have a hard time to infer which directive should be first expand.

Consider this example:

<child>
    <div slot-scope="props" v-for="i in props.items"/>
</child>

We should first expand it to <template slot-scope="props"> ... </template>, right?

But if we add an :slot option, it doesn't seem right.

<child>
    <div slot-scope="props" v-for="i in parentItems" :slot="props[i]"/>
</child>

For this template, it should be compiled to

<template v-for="i in parentItems">
  <template slot-scope="props" :slot="props[i]">
   <div></div>
  </template>
</template>

Update:

maybe we can use a simpler strategy:
always prefer expanding slot-scope over v-for unless there is a :slot binding.

[yyx990803]

The usage here is indeed ambiguous, I also suggest using an explicit <template> wrapper if you need v-for. We should add a warning for this case.

[razbakov]

It looks like warning is not possible to solve in some cases, see #7722