Type {} is not assignable to type PartialDeep<T>

[tommy-mitchell]

Combining PartialDeep with a generic does not accept an empty object initializer:

import type {PartialDeep, UnknownRecord} from 'type-fest';

type Person = {
  name: string;
  age: number;
};

const mock = <T extends UnknownRecord>(mock: PartialDeep<T> = {}): T => mock as T;
//                                     ~~~~~~~~~~~~~~~~~~~~~~~~~
//=> Error (ts2322): Type `{}` is not assignable to type `PartialDeep<T>`

const person = mock<Person>({name: 'Bob'});
//    ^? const person: Person

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[Max10240]

Just my own opinion, it probably has nothing to do with PartialDeep, A simpler example is:

// Type 'string' is not assignable to type 'T'.
//   'string' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'string'.(2322)
const func = <T extends string>(value: T = '') => value;

Because T can be any subtype of UnknownRecord, it could even be never; And if T is never, then mock: PartialDeep<T> is never, in this case {} cannot be assigned to never, so TS made such a complaint.

In addition, the following code is error-free:

import type {PartialDeep, UnknownRecord} from 'type-fest';

// no error here, so "{} is assignable to type PartialDeep<T>"
const foo: PartialDeep<UnknownRecord> = {};