Typescript control flow analysis information is not persisted in Slot props

[mlschmidt366]

Describe the bug

Typescript in svelte already supports control flow analysis when using {#if varName} statements in the html section of a component. The typescript compiler understands that inside the if-block, the variable is neither null nor undefined.

However, when passing the variable to a slot inside the if-block, Typescript shows that the variable might be null or undefined when the slot property is used in another component.

A workaround for my use case, where I am trying to wrap a component whose children should only be populated once the parent component has mounted, there is a workaround to simply set the wrapper type not to be null or undefined, but the typing seems to be incorrect. I believe that might be another bug, which allows two variables that are bound using component binding to be of different types?

Reproduction

https://svelte.dev/repl/e3e71ae0b9714eaf8a03c8c2ca65f507?version=3.59.1

Note: REPL doesn't compile due to lacking TypeScript support, but you can download the code and test it locally by running npx svelte-check or in VS Code using the "Svelte for VS Code" Extension.

Logs

No response

System Info

npm: v9.6.4
typescript: v5.0.4
svelte: v3.59.1
svelte-check: v3.3.2

Severity

annoyance

[jasonlyu123]

Duplicate of sveltejs/language-tools#1832

[mlschmidt366]

Thanks, I wasn't aware of the language-tools repository. There I found also an open issue related to two-way binding type safety, which I am (ab-)using in my current workaround for this issue.