Object.Partial creates wrong type

[iyegoroff]

🐞 Bug Report

Describe the bug

The type created by Object.Partial is weaker than expected

Reproduce the bug

type T = { x: { y: never; z: () => 1 } }
// No error
const err: Object.Partial<T, 'deep'> = { x: { z: () => 'test' } }

Expected behavior

tsc should produce an error:

(property) z?: (() => 1) | undefined
Type '() => string' is not assignable to type '() => 1'.
  Type 'string' is not assignable to type '1'.ts(2322)

Possible Solution

type DeepPartial<T> = T extends Record<string, unknown>
  ? Partial<{ [Key in keyof T]: DeepPartial<T[Key]> }>
  : T

// Error: ...  Type 'string' is not assignable to type '1'.ts(2322)
const ok: DeepPartial<T> = { x: { z: () => 'test' } }

[millsp]

Heya @iyegoroff, thanks for reporting this. It's fixed. It will be out in a few minutes. Thanks!

[iyegoroff]

Looks like it is broken again in v9.6.0

[hjkcai]

I have found another broken case:

Playground Link

import { O } from 'ts-toolbelt'

type A = { foo: unknown }
type B = O.Partial<A, 'deep'>

declare const a: A
const b: B = a

The code above results the following error:

Type 'A' is not assignable to type 'OptionalDeep<A>'.
  Types of property 'foo' are incompatible.
    Type 'unknown' is not assignable to type 'OptionalDeep<unknown> | undefined'.
      Type 'unknown' is not assignable to type 'OptionalDeep<unknown>'.(2322)

Normally unknown can never be passed into OptionalDeep because of the constraint T extends object. However when unknown is a value type of an object, it bypassed the constraint, resulting incorrect type OptionalDeep<unknown>.

IMO unknown may need to have special treatment to avoid this pitfall.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.