Wrong type for object assign array

[Grabauskas]

TypeScript Version: 3.8.0-dev.20191218

Search Terms: Object assign spread array

Expected behavior:

const z: {
    '0': 'a';
    '1': 'b';
    '2': 'c';
};

Actual behavior:

const z: {
    [x: number]: never;
    length: number;
    toString(): string;
    toLocaleString(): string;
    pop(): undefined;
    push(...items: never[]): number;
    concat(...items: ConcatArray<never>[]): never[];
    concat(...items: ConcatArray<...>[]): never[];
    ... 24 more ...;
    [Symbol.unscopables](): {
        ...;
    };
}

Code

const z = { ...["a", "b", "c"] };

Output

"use strict";
const z = Object.assign({}, ["a", "b", "c"]);

Compiler Options

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "useDefineForClassFields": false,
    "alwaysStrict": true,
    "allowUnreachableCode": false,
    "allowUnusedLabels": false,
    "downlevelIteration": false,
    "noEmitHelpers": false,
    "noLib": false,
    "noStrictGenericChecks": false,
    "noUnusedLocals": false,
    "noUnusedParameters": false,
    "esModuleInterop": true,
    "preserveConstEnums": false,
    "removeComments": false,
    "skipLibCheck": false,
    "checkJs": false,
    "allowJs": false,
    "declaration": true,
    "experimentalDecorators": false,
    "emitDecoratorMetadata": false,
    "target": "ES2017",
    "module": "ESNext"
  }
}

Playground Link: Provided

[RyanCavanaugh]

See #9726

[Grabauskas]

Maybe at least Object.assign method can be fixed with some overloads?

For example:

interface ObjectConstructor {
    assign<T, U extends (string | unknown[])>(target: T, source: U): T & Record<number, string>;
    assign<T, U extends (boolean | number)>(target: T, source: U): T;
    assign<T, U extends (null | undefined)>(target: T, source: U): T;
    assign<T, U>(target: T, source: U): T & U;
}

Then it will catch errors:

const a: string[] = Object.assign({}, []); // Type 'Record<number, string>' is missing the following properties from type 'string[]': length, pop, push, concat, and 25 more.
const b: string[] = Object.assign({}, "ASD"); // Type 'Record<number, string>' is not assignable to type 'string[]'.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.