bug: slot fallback broken in components built with 2.6.14 running in 2.6.12

[privatenumber]

Version

2.6.14

Reproduction link

Slot fallback broken running in 2.6.12:
https://jsfiddle.net/hirokiosame/dotsp9ak/

Slot fallback working in 2.6.14:
https://jsfiddle.net/hirokiosame/eb16s0hx/

Steps to reproduce

1. Build the following component with vue-template-compiler v2.6.14:

<template>
	<div>
		test component
		<slot>
			- slot fallback works!
		</slot>
	</div>
</template>

2. Run component in a Vue.js v2.6.12 app:

<template>
    <test-component />
</template>

<script>
import TestComponent from './TestComponent.js';

export default {
  components: {
  	TestComponent,
  },
}
</script>

What is expected?

For a component built with vue-template-compiler v2.6.14 to be backwards compatible with a Vue.js v2.6.12 runtime.

It's not only no longer working, it actually displays "undefined" in place of the slot fallback.

Since there is no new feature being leveraged, I think it's expected for behavior to be preserved.

What is actually happening?

Slot fallback not working in component built with vue-template-compiler v2.6.14, but running in a Vue.js 2.6.12 app

[posva]

This is expected: templates compiled with version 2.x.y can only be used with version 2.x.y or superior. In other words, your user must use the latest Vue too.

[privatenumber]

I would agree if I was leveraging a new feature that's not available in v2.6.12, but I think it's understandable to expect something as fundamental as slots to be backwards compatible.

[posva]

Unfortunately no because that's not how compilation works 😅 . It's a take-all or nothing kind of thing.

There was a fix to slots that created that incompatibility and it has been discussed in other issues already. Sometimes it is possible to make the compilation of templates to be backwards compatible but in this case, the fix (#12014) requires incompatibility.

[privatenumber]

I see. If it's not possible because it's a side-effect to fixing an underlying bug then that's understandable too.

Thanks for explaining and referencing the PR.