DeepPartial does not allow empty object for generics

[mpiniarski]

Description

When the DeepPartial generic argument is generic itself, the resulting type does now allow {} as a value.

Steps to Reproduce

Given the code:

import type { DeepPartial } from "utility-types";

const test = <T extends object>() => {
    const result: DeepPartial<T> = {} //  ❌ type error, "TS2322: Type '{}' is not assignable to type 'DeepPartial'."
    return result;
}

even though it works well for

const result: DeepPartial<{ test: string }> = {}

Typescript cannot limit conditional type in DeepPartial to the case when T extends object even though the passed parameter is declared this way.

Expected behavior

const result: DeepPartial<T> = {}

does not cause a type error.

Suggested solution(s)

The exported DeepPartial should work on objects.

export declare type DeepPartial<T> = {
    [P in keyof T]?: _DeepPartial<T[P]>;
};
/** @private */
export declare type _DeepPartial<T> = T extends Function ? T : T extends Array<infer U> ? _DeepPartialArray<U> : T extends object ? _DeepPartialObject<T> : T | undefined;
/** @private */
export interface _DeepPartialArray<T> extends Array<_DeepPartial<T>> {
}

Project Dependencies

• Utility-Types Version: 3.10.0
• TypeScript Version: 4.9.5

• tsconfig.json:

  
  {
  "extends": "@tsconfig/node16/tsconfig.json",
  "include": ["src/**/*"],
  "compilerOptions": {
  "importsNotUsedAsValues": "error",
  "stripInternal": true,
  
  "declaration": true,
  "sourceMap": true,
  
  "strict": true,
  /* Additional Checks */
  "noUnusedLocals": true,
  "noUnusedParameters": true,
  "noImplicitOverride": true,
  "noImplicitReturns": true,
  "noFallthroughCasesInSwitch": true,
  
  /* Module Resolution Options */
  "moduleResolution": "node",
  "skipLibCheck": false,
  "resolveJsonModule": true
  "esModuleInterop": true
  }
  }

[mpiniarski]

@piotrwitek I'll submit a PR.

[piotrwitek]

Hej @mpiniarski yes feel free to do it, thanks :) Could you also try to add some new test cases to cover that type bug?