Svelte 5 does not allow let bindings on forwarded slots, but Svelte 4 does

[dimfeld]

Describe the bug

Code such as <slot name="a" slot="a" let:abc abc={abc}> throws a compiler error in Svelte 5 that "slot can only receive attributes, not directives". This works fine on Svelte 4.

This is mostly a problem when using old code or libraries that contain this syntax.

Reproduction

Svelte 5 REPL

See Component2.svelte for where the error occurs

Svelte 4 Equivalent REPL that works

Logs

No response

System Info

Svelte REPL

Severity

blocking an upgrade

[mquandalle]

Not exactly the same issue, but #9797 looks similar and might be fixed at the same time.

[dimfeld]

I don't think resolving this one will fix 9797. This one comes up in a different part of the code that is more just around syntax checking, whereas 9797 looks like it comes up in a later compiler phase.