A `store` of type `Readable<number> | undefined` generates `$store: number | undefined` even when inside a guarding `if(store)`

Describe the bug

Let’s say we have a store: Readable<number> | undefined. Inside a guarded if (store)-block, $store should be of type number, but currently it is number | undefined.

To Reproduce

<script lang="ts">
  import { readable, Readable } from "svelte/store";

  let store: Readable<number> | undefined;

  if (!store) {
    store = readable(42);
  } else {
    const storeValue: number = $store;

    // fails with:
    // Type 'number | undefined' is not assignable to type 'number'.
    //   Type 'undefined' is not assignable to type 'number'.ts(2322)
  }
</script>

Expected behavior

The type of $store should be number when guarded with if (store).

System (please complete the following information):

"svelte-check": "^2.3.0"

[derolf]

Btw I think it’s a regression. The problem suddenly appeared in our codebase when I returned from PTO. Some change between middle December and now?

[max24192]

Your example can be rewritten by using $maybeUndefined in the #if check as well which gets rid of the error.

Then, the button would only be shown if it was enabled, because a disabled button returns false. I’d have to write

{#if maybeUndefined && $maybeUndefined !== undefined}

when I want to show a button depending on whether a store was supplied or not.