$props tagged union across multiple props does not type narrow

[cloudymeatball]

Describe the bug

Tagged union props does not type narrow in switch or if.

I am not sure if it was ever supported in svelte 5. Problem appears to go away if instead combining type, properties using a single prop wrapper: {type, properties} then wrapper.type, wrapper.properties etc. So there was not really an equivalent in svelte 4 export let syntax for comparison. I wrote the tagged union types a while ago but only now making use of the inferences or narrowing.

Reproduction

<script lang=ts>
  let {type, properties} : ({
    type: 'student'
    properties: { studentId: number }
  } | {
    type: 'teacher'
    properties: { teacherId: number }
  }) = $props()

  switch (type){
    case 'student':
      console.log(properties?.studentId)
  }

  if (type === 'student'){
    console.log(properties?.studentId)
  }
</script>

// error: Property 'studentId' does not exist on type '{ studentId: number; } | { teacherId: number; }'.

Expected behavior: ts playground

Logs

No response

System Info

svelte^5.0.0
typescript^5.0.0
vite-plugin-svelte^5.0.0

Severity

annoyance

[7nik]

It won't work because it's a typical problem of TS: it doesn't track the relationship between variables.

And this is a correct example of narrowing and not narrowing.

[webJose]

Try let { ...allProps }: Person = $props() or maybe even let props: Person = $props(), although I have never seen the latter anywhere.

[cloudymeatball]

let _props: Person = $props() followed by let {type, properties} = _props does not (seem to) work because it just brings the issue one level down, but as mentioned in bug report _props.type and _props.properties do work.

It appears any, unknown and as just stops the type narrowing sometimes, as demonstrated. With some refactoring I can just bring type into properties. It's tolerable in my case as the refactoring is not too daunting (I hope).

[jasonlyu123]

As pointed out, your expected behaviour is flawed. In the example, the destructured Person is always of the student type. So it's not that the narrowing work but it's already narrowed based on the initial value. In the example @\7nik provided, the as any tells the typescript to not look at the initial value. Not that it stops narrowing. You need to narrow the type for an object, not destructured individual variables.

[cloudymeatball]

Closing because it is consistent with various ts examples, including ones not shown above. For example iterating through an array of Persons and neither if nor switch further narrows the type. This seems beyond scope of svelte.