Version

3.0.5

Steps to reproduce

In any SSR'd application that makes use of slots, you should see the erroneous warning. [Vue warn]: SSR-optimized slot function detected in a non-SSR-optimized render function. You need to mark this component with $dynamic-slots in the parent template.

Solution

On this line: https://github.com/vuejs/vue-next/blob/f81484dbc07734926e47faab07153ea7e6e3b672/packages/runtime-core/src/helpers/renderSlot.ts#L32

A length check is being applied to a function When using length on a function, it will return the number of arguments.

Since the number of arguments is always 4, it will always return 4.

Since the check looks for a length greater than 1, it will always issue a warning.

...Assuming this length check was intended for an array, not a function.

HOWEVER, if this check was intended and there is some magic going on with the number of arguments.. then please document the $dynamic-slots config as nothing is coming up on a search (except other confused users) #2970

Hi, thank you for your interest in Vue, but do you agree that this is a duplicate of #2970?

@HcySunYang sure, close this one or the other to prevent duplication. I just wanted to surface the issue.

Duplicate of #2970?

@HcySunYang can you please close the other ticket in favor of this one? I provide the (easy) fix and also I'm worried that ticket is buried already.

In SSR it creates super annoying output:

@arpowers in that case, it will make more sense if you open the PR with the fix and the test and reference the other issue which was opened first.

@HcySunYang suggest this check is removed since it also causes slots not to work with SSR