TS2322: Type '{}' is not assignable to type 'DeepPartial<T>'

[FrancescoBorzi]

Using ts-essentials version 5.0.0.

I have a function with the following types:

const myFunction = <T extends {}>(someCondition: boolean, object: T): DeepPartial<T> =>
  someCondition ? {} : object;

I know this is a bit weird but this is just to have a minimal bug reproduction: basically my function can return T, a DeepPartial<T> or the empty object {}.

Unfortunately, I get this error:

TS2322: Type '{}' is not assignable to type 'DeepPartial<T>'

However, if I use Partial<T extends {}> I don't get this issue:

const myFunction = <T extends {}>(someCondition: boolean, object: T): Partial<T> =>
  someCondition ? {} : object;

so basically {} can be a Partial<T>, but it can't be a DeepPartial<T>, which is weird.

[quezak]

Confirmed, thanks for finding this!

Looks like this was introduced at v3.0, when the DeepPartial implementation was "turned inside out" in a refactor -- see the commit here

@krzkaczor @FrancescoBorzi can you see which of the new conditions cause DeepPartial to reject {}? also asking @lucifer1004 since you've made that particular refactor

[quezak]

The last version (2.12) that worked with {}, in case someone wants to play around comparing to the current version:

export type DeepPartial<T> = {
  [P in keyof T]?: T[P] extends Primitive
    ? T[P]
    : T[P] extends Function
    ? T[P]
    : T[P] extends Date
    ? T[P]
    : T[P] extends Array<infer U>
    ? Array<DeepPartial<U>>
    : T[P] extends ReadonlyArray<infer U>
    ? ReadonlyArray<DeepPartial<U>>
    : DeepPartial<T[P]>
};

[lucifer1004]

I think there might be some bug related to TypeScript's recursion mechanism when there are conditions.

See the following example:

export type DeepPartial<T> = T extends Builtin
? { [K in keyof T]?: DeepPartial<T[K]> }
: { [K in keyof T]?: DeepPartial<T[K]> };

const myFunction = <T extends {}>(bool: boolean, obj: T): DeepPartial<T> => bool ? {} : obj;
// Type '{}' is not assignable to type 'DeepPartial<T>'.ts(2322)

While

export type DeepPartial<T> = { [K in keyof T]?: DeepPartial<T[K]> };

const myFunction = <T extends {}>(bool: boolean, obj: T): DeepPartial<T> => bool ? {} : obj;

[quezak]

Should we file an issue to TS then? Can we somehow work around this issue? (well, apart from going back to the form before the refactor I linked)

[Beraliv]

The side effect of conditional types, the issue to TypeScript - microsoft/TypeScript#37524