DeepReadonly type fails to compile on typescript 3.1.1+

[masterkidan]

TypeScript Version: 3.1.1+

Search Terms: DeepReadonly

Code

export type Primitive = string | number | boolean | undefined | null | Function | symbol;
export type DeepReadonly<T> =
 T extends [infer A] ? DeepReadonlyObject<[A]> :
 T extends [infer A, infer B] ? DeepReadonlyObject<[A, B]> :
 T extends [infer A, infer B, infer C] ? DeepReadonlyObject<[A, B, C]> :
 T extends [infer A, infer B, infer C, infer D] ? DeepReadonlyObject<[A, B, C, D]> :
 T extends [infer A, infer B, infer C, infer D, infer E] ? DeepReadonlyObject<[A, B, C, D, E]> :
 T extends [infer A, infer B, infer C, infer D, infer E, infer F] ? DeepReadonlyObject<[A, B, C, D, E, F]> :
 T extends [infer A, infer B, infer C, infer D, infer E, infer F, infer G] ? DeepReadonlyObject<[A, B, C, D, E, F, G]> :
 T extends Map<infer U, infer V> ? ReadonlyMap<DeepReadonlyObject<U>, DeepReadonlyObject<V>> :
 T extends Set<infer U> ? ReadonlySet<DeepReadonlyObject<U>> :
 T extends Promise<infer U> ? Promise<DeepReadonlyObject<U>> :
 T extends Primitive ? T :
 T extends (infer A)[] ? DeepReadonlyArray<A> :
 DeepReadonlyObject<T>;

interface DeepReadonlyArray<T> extends ReadonlyArray<DeepReadonly<T>> {}
type DeepReadonlyObject<T> = { readonly [P in keyof T]: DeepReadonly<T[P]> }

Expected behavior:
This used to work as of typescript 3.0.1

Actual behavior:
In typescript 3.1.1 a new error is thrown saying that the type is cyclical
In typescript 3.2+ the above error, instead ends up casting the type as any
Playground Link:
Here's a gist with tests for the type
https://gist.github.com/masterkidan/7322752f569b1bba53e0426266768623

Playground link

Related Issues:
#27421
#13923
#26236

[masterkidan]

I understand this may be by design, but is there a way to achieve the same with newer versions of typescript?

[Dean177]

Mukundan Kidambi (@masterkidan) looks like this is due to the tuple types, without those your code works as expected
Playground link

[masterkidan]

Dean Merchant (@Dean177) : Thanks!, is there a way to make this work without removing support for Tuples?

[dragomirtitian]

Actually I think you are better off wating for 3.4 and Improved support for read-only arrays and tuples (#29435). With this new feature mapped types will apply the readonly modifier to tuples and arrays and you will not need any special handling for them. This will work as expected (as far as I can tell) in 3.4 (you can try it out with npm install typescript@next:

export type DeepReadonly<T> =
 T extends Map<infer U, infer V> ? ReadonlyMap<DeepReadonlyObject<U>, DeepReadonlyObject<V>> :
 T extends Set<infer U> ? ReadonlySet<DeepReadonlyObject<U>> :
 T extends Promise<infer U> ? Promise<DeepReadonlyObject<U>> :
 T extends Function ? T : DeepReadonlyObject<T> 

type DeepReadonlyObject<T> = { readonly [P in keyof T]: DeepReadonly<T[P]> }

[aleclarson]

Immer has a "deep readonly" type called Immutable (see here) that works fine in 3.1+

(No support for Map, Set, or Promise like the OP, but you can add that yourself rather easily)