Enhance recursive type

[himself65]

TypeScript Version: 3.7.0-dev.20191018

Search Terms:

• recursive type
• Recursive type references #33050

Code
Example:

type UnwrapRef<T> = T extends Ref<infer V>
  ? UnwrapRef<V>  // <-- this line will throw error
  : T extends Array<infer V>
    ? Array<UnwrapRef<V>>
    : T extends BailTypes
      ? T // bail out on types that shouldn't be unwrapped
      : T extends object ? { [K in keyof T]: UnwrapRef<T[K]> } : T

Expected behavior:

export type UnwrapRef<T> = {
  ref: T extends Ref<infer V> ? UnwrapRef<V> : T
  array: T extends Array<infer V> ? Array<UnwrapRef<V>> : T
  object: { [K in keyof T]: UnwrapRef<T[K]> }
  stop: T extends Ref<infer V> ? UnwrapRef<V> : T
}[T extends Ref
  ? 'ref'
  : T extends Array<any>
    ? 'array'
    : T extends BailTypes
      ? 'stop' // bail out on types that shouldn't be unwrapped
      : T extends object
        ? 'object' : 'stop']

they are the same effects, but why couldn't I code like the first example?

Actual behavior:

Error:(14, 5) TS2315: Type 'UnwrapRef' is not generic.

Playground Link:

https://gist.github.com/Himself65/dea93908f17d25093938fcedc68eabd3

[nmain]

Possibly related to #33868

[HerringtonDarkholme]

This is expected behavior. Conditional type alone doesn't guarantee recursive type alias. Type alias must have one layer of indirection, e.g., as field of interface, array/tuple's item type, as listed in #33050 .

In this example, UnwrapRef directly uses itself in the true branch of conditional type. This fails to meet the "indirection" requirement of recursive type alias.

Allowing recursion in conditional type is proposed in #26980 .

Though, I think the error reported here should be "cyclic type alias" rather than "type is not generic".

[RyanCavanaugh]

Thanks @nmain / @HerringtonDarkholme ♥