Unable to overload arrow function in module augmentation

[dotansimha]

I'm trying to overload an existing arrow function in a module, exported as const. It works perfectly fine with overrides for function, class and interface, but failing for arrow function declared as const.

Also tried with:

    export interface myVar {
        (param: number): number
    }

It works, but then the arrow function become unusable because it treats it as interface and it's not callable.

TypeScript Version: 3.9.2

Search Terms: const, module augmentation, declaration merging, overload, block-scope variable

Expected behavior:

Allow to override block-scoped variables if it's referring to an arrow function. Just like a function.

Actual behavior:

Cannot redeclare block-scoped variable 'myVar'.(2451)

Related Issues:

• Merging constants with namespaces #18163 (maybe related)

Code

declare module 'test' {
    // export function works
    export function myFunc(param: string): string;

    // export const doesn't work
    export const myVar: (param: string) => string;
}

declare module 'test' {
    export function myFunc(param: number): number;
    export const myVar: (param: number) => number;
}

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]

This is the intended behavior; consts are assumed to be in conflict but multiple function declarations adds new overloads.

[dotansimha]

Thanks @RyanCavanaugh !
Maybe I'm missing something here, but const that point to an arrow function is basically a function...

[orta]

The goal of that message (I assume) is to match runtime behavior where the function declaration acts like var vs the const which never allow overriding:

❯ node
Welcome to Node.js v13.6.0.
Type ".help" for more information.
> function a() {}
undefined
> function a() {}
undefined

> const b = () => {}
undefined
> const b = () => {}
Uncaught SyntaxError: Identifier 'b' has already been declared

[typescript-bot]

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