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