Generator functions cannot be overloaded

[birtles]

TypeScript Version: 3.9.2

Search Terms: overload generator

Expected behavior: It should be possible to overload generators (including async generators) just as it is possible to overload functions.

Actual behavior: TS1222 An overload signature cannot be declared as a generator is returned.

Related Issues: #25710

Code

// NOT using generators
function yer(type: 'a'): Array<number>;
function yer(type: 'b'): Array<string>;

function yer(type: 'a' | 'b'): Array<any> {
    if (type === 'a') {
        return [0, 1, 2];
    }
    return ['hello', 'there'];
}

// Using generators
function* bleh(type: 'a'): IterableIterator<number>;
function* bleh(type: 'b'): IterableIterator<string>;

function* bleh(type: 'a' | 'b'): IterableIterator<any> {
    if (type === 'a') {
        yield 0;
        yield 1;
        yield 2;
        return;
    }

    yield 'hello';
    yield 'there';
}

Output

"use strict";
function yer(type) {
    if (type === 'a') {
        return [0, 1, 2];
    }
    return ['hello', 'there'];
}
function* bleh(type) {
    if (type === 'a') {
        yield 0;
        yield 1;
        yield 2;
        return;
    }
    yield 'hello';
    yield 'there';
}

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

[ilogico]

As far as I can tell, generator functions don't have a representation in the type system.
They are functions that return generators, just like async functions are merely functions that return promises.

That being said, you can overload them normally. Just remove the * like this.

[birtles]

That being said, you can overload them normally. Just remove the * like this.

Oh, interesting!

Is there a way to do the same for async generators?

The specific code I am trying to write is as follows:

async function* bleh(type: 'a'): AsyncIterableIterator<number>;
async function* bleh(type: 'b'): AsyncIterableIterator<string>;

async function* bleh(type: 'a' | 'b'): AsyncIterableIterator<any> {
    if (type === 'a') {
        yield 0;
        yield 1;
        yield 2;
        return;
    }

    yield 'hello';
    yield 'there';
}

[birtles]

Is there a way to do the same for async generators?

Nevermind, I realized I can overload this by removing both async and *:

function bleh(type: 'a'): AsyncIterableIterator<number>;
function bleh(type: 'b'): AsyncIterableIterator<string>;

async function* bleh(type: 'a' | 'b'): AsyncIterableIterator<any> {
    if (type === 'a') {
        yield 0;
        yield 1;
        yield 2;
        return;
    }

    yield 'hello';
    yield 'there';
}

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow.

[AgainPsychoX]

Why can't we allow sugar syntax? I wish it would work with both star and async...