Overload gets lost in mapped type with conditional type

[felixfbecker]

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms: mapped type overload

Code

interface Overloads {
  foo(a: string): void;
  foo(a: number, b: string): void;
}

/** Converts all properties of an object to Promises and all methods to return Promises */
type ProxiedObject<T> = {
  [P in keyof T]: T[P] extends (...args: infer Arguments) => infer R
    ? (...args: Arguments) => Promise<R>
    : Promise<T[P]>
};

declare let x: ProxiedObject<Overloads>;
x.foo("abc"); // Error: [ts] Expected 2 arguments, but got 1. [2554]
x.foo(123, "abc");

Expected behavior:
No error, overload should be maintained.

This makes it impossible to use this pattern with popular types that contain overloads, like Rx Observable pipe()/subscribe().
The ProxiedObject type is used in https://github.com/GoogleChromeLabs/comlink.

Actual behavior:
Overload gets lost, compile error when trying to call the first overload.

Playground Link: link

[jack-williams]

When inferring from a type with multiple call signatures (such as the type of an overloaded function), inferences are made from the last signature (which, presumably, is the most permissive catch-all case). It is not possible to perform overload resolution based on a list of argument types (this would require us to support typeof for arbitrary expressions, as suggested in #6606, or something similar).

From #21496. I suspect this is probably a design limitation.

[felixfbecker]

But that was before we had tuple types. This overload:

interface Overloads {
  foo(a: string): void;
  foo(a: number, b: string): void;
}

is equivalent to

interface NoOverloads {
  foo(...args: [string] | [number, string]): void;
}

which works correctly. I would use that workaround, but I don't have control over libraries that use overloads. I would at least expect the compiler to treat the above interface equivalently to the below.

If the return type is different, it's also possible with conditional types:

interface Overloads {
  foo(a: string): string;
  foo(a: number, b: string): number;
}

interface NoOverloads {
  foo<P>(...params: P): P extends [number, string] ? number : string;
}

So I don't see a reason why overloads would still be impossible for the compiler to handle

[jack-williams]

The tuple approach seems plausible; perhaps that could be a proposal included in the issue?

The conditional type approach elicits further problems because generic parameters do not get unified.

[nixxquality]

I would just like to point out that

interface NoOverloads {
  foo(...args: [string] | [number, string]): void;
}

const Bar: NoOverloads = {
    foo: function (a, b) {
        return;
    }
}

actually does not work currently.
Check it out on the playground - a and b both get the type number | string

I'm personally very interested in seeing typeable overloads. I'm struggling to get TypeScript to accept my current typings for one of my projects: here it is.

[brunolemos]

Not sure if it's the same issue but I came here to report this behavior:

type Post = { body: string }
type Member = { name: string }

type methods = {
    on(event: 'new_post', payload: Post): void
    on(event: 'member_add', payload: Member): void
}

type Params = Parameters<methods['on']>
// Expected Params type: ["new_post", Post] | ["member_add", Member]
// Current behavior: ["member_add", Member] (only gets the last one)

Playground

Related use case: probot/probot#858

[rhyek]

Any news on this? Just hit this bug myself.

[ClickerMonkey]

This limitation has been a thorn in my side for multiple projects. Can anything be done to assist?

[dragomirtitian]

@ClickerMonkey For non generic overloaded functions you can use the approach described here to improve the situation

[graup]

I stumbled upon this issue while trying to generate generic overloads programmatically. Here's a workaround:

Unions are not treated like overloads, but intersections are. So, we just need to find a way to create an intersection of interfaces containing the overloads. We can do that by wrapping the function signatures in interfaces.

Playground example

[LukasBombach]

Thank you @graup for the workaround and the example

I made your Playground simpler so it's easier to follow the idea:

Simplified Playground example

[unional]

A very simple example

But that was before we had tuple types.

Hi @felixfbecker!
Yep~. while inference works, sometime is so complicated that overload is a much better approach. e.g., I'm working on this type:

// still work in progress, I'm trying to further simplify it.
export type SetState<T> = undefined extends T ? {
  (value: T | ((draft: T) => T | void), meta?: { logger?: Logger }): T,
  (value: undefined | typeof nothing | ((draft: T) => typeof nothing), meta?: { logger?: Logger }): undefined,
  (value: (draft: T) => Promise<T | void>, meta?: { logger?: Logger }): Promise<T>,
  (value: (draft: T) => Promise<typeof nothing>, meta?: { logger?: Logger }): Promise<undefined>
} : {
  (value: T | ((draft: T) => T | void), meta?: { logger?: Logger }): T,
  (value: (draft: T) => Promise<T | void>, meta?: { logger?: Logger }): Promise<T>,
}

It is very difficult to convert that to use only inference. 🍻

[unional]

Since we can't use tuple in rest param type, I don't think we can use conditional type to replace function overloads.

e.g.

type F = {
  () => number,
  (value: string) => string
}

// vs
type F = <P extends Array<string>>(...args: P): P extends [] ? number : (P extends [string] ? string : never)

// this won't work
<P extends [] | [string]>(...args: P): ...

let f: F
f('a', 'b') // function overload prevents this, but conditional type can't

also, user can't easily see what params f() can take until they try it out.
From the type annotation it just show as (...args: P): ....
User have to read through the whole conditional type and understand what it means,
and the arguments are named as arg_0, arg_1 etc.

As compare to function overload where you can name the arguments properly.