Svelte-check or TS error when using let with named slots

[tyler-boyd]

Describe the bug

Svelte-check, or maybe TS, seem to be broken when using let: variables with named slots. If I do this, then the code explodes at runtime but there are no svelte-check errors:

<Wrapper let:foo>
  <div class:foo slot="title">title2</div>
  <div class:foo>{foo}</div>
</Wrapper>

(^error: foo is not defined in <unknown> in __wrapper.svelte)

while if I do this, the code works at runtime, but svelte-check fails

<Wrapper let:foo>
  <div class:foo slot="title" let:foo>title2</div>
  <div class:foo>{foo}</div>
</Wrapper>

Please let me know if I'm doing something wrong 😅 currently my poor app is broken and I don't know how to fix it!

Reproduction

See https://github.com/tyler-boyd/svelte-ts-bug-repro. I just ran npm create vite@latest and created App.svelte and Wrapper.svelte to show this minimal reproduction. If you run npm run check, you'll see it fails, plus errors are shown in VS code.

> svelte-check-bug-repro@0.0.0 check
> svelte-check --tsconfig ./tsconfig.app.json && tsc -p tsconfig.node.json

Loading svelte-check in workspace: /Users/Tyler.Boyd/work/svelte-check-bug-repro
Getting Svelte diagnostics...

/Users/Tyler.Boyd/work/svelte-check-bug-repro/src/App.svelte:6:35
Error: Property 'foo' does not exist on type '{}'. (js)
<Wrapper let:foo>
  <div class:foo slot="title" let:foo>title2</div>
  <div class:foo>{foo}</div>

====================================
svelte-check found 1 error and 0 warnings in 1 file

Logs

System Info

System:
    OS: macOS 15.5
    CPU: (10) arm64 Apple M1 Pro
    Memory: 650.94 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.12.2 - ~/.asdf/installs/nodejs/20.12.2/bin/node
    npm: 10.5.0 - ~/.asdf/installs/nodejs/20.12.2/bin/npm
    bun: 1.1.15 - ~/.bun/bin/bun
  Browsers:
    Chrome: 138.0.7204.169
    Safari: 18.5
  npmPackages:
    svelte: ^5.35.5 => 5.36.16

Severity

blocking an upgrade

[tyler-boyd]

Hmm, I think I just did a bad job reading the error message. The error I was seeing is caused by me trying to bind a variable that isn't actually defined by that slot. So the compiler was correctly trying to protect me from a mistake.

There is still a bug/gap where svelte-check doesn't identify this code as invalid (and thus it passes svelte-check but explodes at runtime), but that's not a blocker.

<Wrapper let:foo>
  <div class:foo slot="title">title2</div>
  <div class:foo>{foo}</div>
</Wrapper>

I will close this as the reported issue is not actually an issue.