[v2] broken functional components with multiple root

[lancetharper]

Describe the bug
Functional components that render multiple root modules no longer work

To Reproduce
CodeSandbox

Steps to reproduce the behavior:

Component functional-a renders a single element (and is imported automatically).

Component functional-b throws an error about multiple root elements when imported automatically. If you uncomment the manual import and manually include the component in pages/index.vue it works correctly.

Expected behavior
Functional components can render multiple roots

Additional context
Discovered when I updated from nuxt 2.14.12 to 2.15.4

[NozomuIkuta]

I just folked and played around with the demo.

This bug can be avoided by setting components.loader: true.

CodeSandbox

Please note that there are some limitations or trade-off of lower performance in development mode, according to the authors.