TS 3.9 - No overload matches this call

[alan-agius4]

TypeScript Version: 3.9.1-rc

Search Terms:
No overload matches this call

Code

export interface SyncNgccOptions {
    basePath: string;
    targetEntryPointPath?: string;
    async?: false;
}

export type AsyncNgccOptions = Omit<SyncNgccOptions, 'async'> & { async: true };

export function process(options: AsyncNgccOptions): Promise<void>;
export function process(options: SyncNgccOptions): void;
export function process(options: AsyncNgccOptions | SyncNgccOptions): void | Promise<void> {
    return mainNgcc(options);
}


export function mainNgcc(options: AsyncNgccOptions): Promise<void>;
export function mainNgcc(options: SyncNgccOptions): void;
export function mainNgcc(options: AsyncNgccOptions | SyncNgccOptions): void | Promise<void> {
}

Expected behavior:
No error as per TS 3.8

Actual behavior:
Error

No overload matches this call. Overload 1 of 2, '(options: AsyncNgccOptions): Promise', gave the following error. Argument of type 'SyncNgccOptions | AsyncNgccOptions' is not assignable to parameter of type 'AsyncNgccOptions'. Type 'SyncNgccOptions' is not assignable to type 'AsyncNgccOptions'. Type 'SyncNgccOptions' is not assignable to type '{ async: true; }'. Types of property 'async' are incompatible. Type 'false | undefined' is not assignable to type 'true'. Type 'undefined' is not assignable to type 'true'. Overload 2 of 2, '(options: SyncNgccOptions): void', gave the following error. Argument of type 'SyncNgccOptions | AsyncNgccOptions' is not assignable to parameter of type 'SyncNgccOptions'. Type 'AsyncNgccOptions' is not assignable to type 'SyncNgccOptions'. Types of property 'async' are incompatible. Type 'true' is not assignable to type 'false | undefined'.

Playground Link: https://www.typescriptlang.org/v2/en/play?ts=3.9.0-beta#code/KYDwDg9gTgLgBASwHY2FAZgQwMbDgZQE8lsA5Ac22wHkwYEIkBnOAbwCg4u4AjTJ4AAVMMABYAuOExhRk5ANyduMTFHLAYAURRRCgiMhjCxAfknTZSBUq79i2M3CwAbAYoC+7dqEiw4MQjA8AEEmewoqWnpGFgBeOGoAWwQYAB4iEgiaOgZmABo4AHI7EkKAPjgAMjY4EuxJGQBXPHdFb3BoeHRGkmikODAoCFwmJgAKCByYyVDwymy+pgBKSUEh5IFUgDcDABMyxR9Opx7sPoGhkfHJxckMsnmo3OXJHYRdw46-bt7ci+HgKMJlNmDMwplHiCWAAfAhzSJQlZwN67OCwtYQDbAbZ7CocbhwKAaRpQfqJTDILLAxZLDxedq+Lqnc7kynzanPMHwhbPJEYrE494HBnHH5nP6spBUm6cuEQhE0157T6Mk6-RhwSXSqFc+U8mJouUPBW8pXvQ38hCbFF49ieIA

Related Issues:

[kitsonk]

It is a breaking changed covered in #37195. async? being optional is causing your problem. It was unsound code before that is now being caught.

[petebacondarwin]

This convoluted typing was created because we could not work out a better way to support a function that if invoked with async: true would return a Promise or void otherwise.

@kitsonk - do you have a suggestion for how we can support this scenario?

[gkalpak]

I think we just need to define one extra overload (although I would expect TS to be able to infer that on its own 😞):

 export function mainNgcc(options: AsyncNgccOptions): Promise<void>;
 export function mainNgcc(options: SyncNgccOptions): void;
+export function mainNgcc(options: AsyncNgccOptions | SyncNgccOptions): void | Promise<void>;
 export function mainNgcc(options: AsyncNgccOptions | SyncNgccOptions): void | Promise<void> {
   // ...
 }

See https://www.typescriptlang.org/v2/en/play?ts=3.9.0-beta#code/KYDwDg9gTgLgBASwHY2FAZgQwMbDgZQE8lsA5Ac22wHkwYEIkBnOAbwCg4u4AjTJ4AAVMMABYAuOExhRk5ANyduMTFHLAYAURRRCgiMhjCxAfknTZSBUq79i2M3CwAbAYoC+7dqEiw4MQjA8AEEmewoqWnpGFgBeOGoAWwQYAB4iEgiaOgZmABo4AHI7EkKAPjgAMjY4EuxJGQBXPHdFb3BoeHRGkmikODAoCFwmJgAKCByYyVDwymy+pgBKSUEh5IFUgDcDABMyxR9Opx7sPoGhkfHJxckMsnmo3OXJHYRdw46-bt7ci+HgKMJlNmDMwplHiCWAAfAhzSJQlZwN67OCwtYQDbAbZ7CocbhwKAaRpQfqJTDILLAxZLDxedq+Lqnc7kynzanPMHwhbPJEYrE494HBnHH5nP6spBUm6cuEQhE0157T6Mk6-RhwSXSqFc+U8mJouUPBW8pXvQ38hCbFHCo7fZkSilS9ky6ZwWZ6p4G2H3LJe5hIlEW9ZW7E2tjsTzsbAxeBiK0ASSYADU9nB4lr2aw+AJjBIiugIBBCu5adHY-5RImU3sAEzpzVOqnZ-hCET5wqF4sFOqSFwCUuKGPMONVphJy0CBuZqhjFu59uSTtFwo98H1fxQZqDrzD6SV6up97BVwQBuDAFA+dtsRLvhQEtlvejw91k9MM-xC9XOc5m8d+9V1qdc+0wVxgB3Z8D3HJhJ2Ad9P3+H9rzzO9VCA3tN23MsgA.

[DanielRosenwasser]

I'm tempted to mark Working as Intended but I'm not totally clear exactly why Omit<..., "async"> & { async: true } causes issues. @ahejlsberg care to weigh in?

[kitsonk]

@DanielRosenwasser not to step on Ander's toes, but I believe it is very intentional, because #37195 was specifically to fix unsafe situations with Omit and intersection types brought up in #36604.

[DanielRosenwasser]

Oh, I see - I wasn't looking at where the error was being issued. This makes sense.

@gkalpak technically you could condense the whole thing down to

// Overload
export function process<T extends AsyncNgccOptions | SyncNgccOptions>(options: T):
  T extends AsyncNgccOptions ? Promise<void> : void;

// Implementation
export function process(options: AsyncNgccOptions | SyncNgccOptions): void | Promise<void> {
    return mainNgcc(options);
}

[alan-agius4]

Thanks guys for your input.

[gkalpak]

Thx, @DanielRosenwasser!

FWIW, I still find overloads more readable (and wish TypeScript would recognize the 3rd overload signature from the implementation):

function process(options: SyncNgccOptions): void;
function process(options: AsyncNgccOptions): Promise<void>;
function process(options: SyncNgccOptions | AsyncNgccOptions): void | Promise<void> {
    // ...
}