Why the line 23 not `satisfies` the type? | TypeScript Community | Page 1

pure cragBOT Jun 4, 2023, 6:39 AM

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SettingDust#4766

Playground Link

hazy bramble Jun 4, 2023, 6:41 AM

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type Primitive =
    | null
    | undefined
    | string
    | number
    | boolean
    | symbol
    | bigint;
type BuiltIns = Primitive | Date | RegExp;
type Cloneable = Record<PropertyKey, unknown> | BuiltIns | Iterable<Cloneable> | Cloneable[]
type ClonedIterable<T extends Iterable<unknown>> = T extends Iterable<infer R>
  ? R extends Cloneable
    ? Iterable<Cloned<R>>
    : never
  : never
type Cloned<T> = T extends Iterable<unknown> ? ClonedIterable<T> : T

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function cloneIterable<T extends Cloneable>(value: Iterable<T>) {
  const iterable = (function* () {
    for (const item of value) yield item as Cloned<T>
  })() as Iterable<Cloned<T>>

  return iterable satisfies ClonedIterable<typeof value>
}

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The satisfies at the end of the function cloneIterable failed

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Type 'Iterable<Cloned<T>>' does not satisfy the expected type 'T extends Cloneable ? Iterable<Cloned<T>> : never'.

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But why? T should extends Cloneable

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Why the line 23 not satisfies the type?

vale cipher Jun 6, 2023, 2:04 PM

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i think it's because you have a generic conditional type involved. TS can't really analyze whether values are assignable to such types. even relatively simple things like this fail to typecheck:

#Why the line 23 not `satisfies` the type?