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"...args: infer" on an overloaded function chooses only one function head #26136
Description
TypeScript Version: 3.1.0-dev.20180801
Search Terms: overload, infer, function heads, tuple, rest parameter
Code
interface Foo {
(a: string, b: number, c: boolean): void;
(a: number, b: string, c: boolean): void;
}
type Args<F> = F extends (...args: infer T) => any ? T : never;
let x: Args<Foo> = [42, 'hello world', true]; // ✅
x = ['hello world', 42, true]; // 💥Expected behavior:
Ideally, I would hope for the typeof x to be
let x: [string, number, boolean] | [number, string, boolean]although not entirely correct, I would even take a type wide enough to accomodate both function heads.
let x: [string|number, number|string, boolean]Actual behavior:
The type of x ends up being
let x: [number, string, boolean]
Activity
mike-north commentedon Aug 2, 2018
Closing as duplicate of Brian Terlson (@bterlson)'s #26132
bterlson commentedon Aug 2, 2018
Mike North (@mike-north) is this a duplicate, though? #26132 doesn't have overloads, and arguments list inference of
fngives something similar to what you request here:mike-north commentedon Aug 2, 2018
Good catch, I must have read your issue too quickly. Maybe this should be un-tagged as duplicate
RyanCavanaugh commentedon Aug 7, 2018
Discussed for a while yesterday. This looks reasonably straightforward if you're just capturing the entire parameter list into a single spread, but quickly turns into a bunch of really complicated questions once you do anything else
type Args<F> = F extends (n: number, ...args: infer T) => any ? T : never;be? Would it be sound to just capture the matching overloads, or not?Tinto a different argument list? Potentially a combinatorially explosive expansion in to overloads?Overall too complex unless there are some important use cases that appear.
spiritwindy commentedon Aug 15, 2018
Ryan Cavanaugh (@RyanCavanaugh)
some example like
Function.prototype.call
Function.prototype.apply
Function.prototype.bind