Type guards working like a charm inside Svelte templates

Guards.svelte

<script lang="ts">
  type Pet = Fish | Bird;

  interface Fish {
    type: "fish";
    isLarge: boolean;
  }

  interface Bird {
    type: "bird";
    isFast: boolean;
  }

  // If type guard is removed (`is ...`), the template below doesn't pass `svelte-check`
  function isFish(pet: Pet): pet is Fish {
    return pet.type === "fish";
  }

  function isBird(pet: Pet): pet is Bird {
    return pet.type === "bird";
  }

  let pet: Pet = {
    type: "fish",
    isLarge: true,
  };
</script>

{#if isFish(pet)}
  <p>a {pet.isLarge ? 'large' : 'small'} fish</p>
{/if}
{#if isBird(pet)}
  <p>a {pet.isFast ? 'fast' : 'slot'} bird</p>
{/if}