TypeScript: DeepPartial not working for union of string and string literals

[fluffy-heinzelman]

Description

Consider the following unions, where union B is designed to provide intellisense code completion (in contrast to A, which is narrowed down to only string by the TS compiler).

If DeepPartial is applied on unions like B we get an TS error when we try to assign an arbitrary string value.

type A = 'foo' | 'bar' | string;
type B = 'foo' | 'bar' | (string & {});

type Key = 'x' | 'y' | 'z';

type ADict = DeepPartial<Record<Key, A>>;
type BDict = DeepPartial<Record<Key, B>>;

// - no code completion for values (as expected)
// - no error (as expected)
const a: ADict = {
    x: 'foo',
    y: 'barrel'
};

// - code completion for values (as expected)
// - TS error for any other string value than `foo` and `bar` (NOT expected)
const b: BDict = {
    x: 'foo',
    y: 'barrel' // TS error
};

Steps to Reproduce

Have a look at this Codesandbox:
https://codesandbox.io/s/issue-demo-utility-types-deeppartial-5zwnw?file=/src/index.ts

Expected behavior

DeepPartial not breaking unions of string and string literals.

Suggested solution(s)

DeepPartial conditional expressions could check for Primitive.

type DeepPartial<T> = 
    T extends Primitive ? Partial<T> : // FIX
    T extends Function ? T : 
    T extends Array<infer U>? DeepPartialArray<U> : 
    T extends object ? DeepPartialObject<T> : 
    T | undefined;

interface DeepPartialArray<T> extends Array<DeepPartial<T>> {}
type DeepPartialObject<T> = {
    [P in keyof T]?: DeepPartial<T[P]>;
};

Project Dependencies

• Utility-Types Version: 3.10.0
• TypeScript Version: 4.1.2
• tsconfig.json:

{
  "compilerOptions": {
    "target": "es5",
    "noImplicitReturns": true,
    "noImplicitThis": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "sourceMap": true,
    "moduleResolution": "node",
    "allowJs": true,
    "noEmit": true,
    "strict": true,
    "esModuleInterop": true,
    "isolatedModules": false,
    "allowSyntheticDefaultImports": true
  }
}