Function argument inference only handles one overload

[JakeTunaley]

TypeScript Version: 3.1.0-dev.20180821

Search Terms: function argument infer overload return type rest tuple

Code

function foo (a: string): string;
function foo (a: number, b: string): boolean;
function foo (a: boolean, ...b: object[]): object;
function foo (...args: any[]): any {
}

type A = ReturnType<typeof foo>;

Expected behavior:

Type A is a union of the return types of every overload, or an error along the lines of "cannot infer things about an overloaded function" is generated.

Actual behavior:

Type A is object.

This also happens when inferring function params:

type Parameters<T extends (...args: any[]) => any> = T extends (...args: infer U) => any ? U : never;

type B = Parameters<typeof foo>; // [boolean, ...object[]]

Playground Link:

http://www.typescriptlang.org/play/#src=function%20foo%20(a%3A%20string)%3A%20string%3B%0D%0Afunction%20foo%20(a%3A%20number%2C%20b%3A%20string)%3A%20boolean%3B%0D%0Afunction%20foo%20(a%3A%20boolean%2C%20...args%3A%20object%5B%5D)%3A%20object%3B%0D%0Afunction%20foo%20(...args%3A%20any%5B%5D)%3A%20any%20%7B%0D%0A%7D%0D%0A%0D%0Atype%20A%20%3D%20ReturnType%3Ctypeof%20foo%3E%3B%0D%0A%0D%0Atype%20Parameters%3CT%20extends%20(...args%3A%20any%5B%5D)%20%3D%3E%20any%3E%20%3D%20T%20extends%20(...args%3A%20infer%20U)%20%3D%3E%20any%20%3F%20U%20%3A%20never%3B%0D%0A%0D%0Atype%20B%20%3D%20Parameters%3Ctypeof%20foo%3E%3B%20%2F%2F%20%5Bboolean%2C%20...object%5B%5D%5D

[RyanCavanaugh]

See comments in #21847 - overload resolution in conditional types is not supported yet

[bdpartridge]

I also encountered this issue while working with a partially-applied version of a hyperscript-style function. I was able to work around it by specifying the last overload of the Hyperscript function type to be a less specific catch-all.

declare function partial<Arg1, Args extends any[], Ret>(
  func: (arg1: Arg1, ...args: Args) => Ret,
  arg1: Arg1,
): (...args: Args) => Ret;

interface Hyperscript {
  (selector: string, ...children: Children[]): Vnode;
  (selector: string, attrs: Attributes, ...children: Children[]): Vnode;
  (selector: string, ...args: [Attributes, ...Children[]] | Children[]): Vnode;
}

type Attributes = Record<string, any>;

type Children = Vnode | string | number | boolean | null | undefined;

interface Vnode {
  tag: string;
  // other props omitted for brevity
}

declare const h: Hyperscript;

// plain hyperscript function
h("div", { id: "foo" }, "foo", h("div", { id: "bar" }, "bar"));
h("div", "foo", "bar");

// partially-applied hyperscript function
const div = partial(h, "div");
div({ id: "foo" }, "foo", div({ id: "bar" }, "bar"));
div("foo", "bar");

This may not help in all cases where this is an issue, but I thought I'd share just in case others might find this useful.

[ggoodman]

I was hoping to leverage the existing typing on known event names in common node EventEmitters with the incoming addition of once(ee: EventEmitter, event: string): Promise<any[]> to the core library.

I took the the following approach in the playground and that led me to adding in my use-case to this issue.

interface EE {
  on(event: string, handler: () => void): void;
}

interface Request extends EE {
  on(foo: 'error', handler: (error: Error) => void): void;
  on(foo: 'data', handler: (data: string) => void): void;
}

type HandlerForEvent<T extends EE, E extends string> = T extends { on: (event: E, handler: infer H) => void } ? H : never;

type ErrorHandler = HandlerForEvent<Request, 'error'>;
type ErrorHandlerArgs = Parameters<ErrorHandler>;
// Types as never

type DataHandler = HandlerForEvent<Request, 'data'>;
type DataHandlerArgs = Parameters<DataHandler>;
// Types as [string] !!

If you reverse the order of the Request#on overloads, the never will swap.

It would be neat if the arguments for overloaded methods were treated as a union of tuples according to the overloads.

[gogoyqj]

well, i have an idea

type GetArgumentsType<Delay> =
    | (Delay extends (one: infer ONE) => any ? [ONE] : never)
    | (Delay extends (...args: infer MANY) => any ? MANY : never);

this worked for me in some case for infer arguments

[buschtoens]

@RyanCavanaugh Is this closed as "won't fix" or was the issue resolved?

[RyanCavanaugh]

Design limitation means "No apparent way of fixing this exists (short of wholescale redesign)". "Won't Fix" would mean "We could fix it but choose not to".

[buschtoens]

I see. Thanks for clarifying! 👌🏼