Optional parameter needs to be provided

[gulewei]

🔎 Search Terms

"ts2554", "optional parameter", "overloads"

🕗 Version & Regression Information

• This changed between versions ______ and _______
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Common "Bugs" That Aren't Bugs___
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/JYOwLgpgTgZghgYwgAgKIDcLgLJwA7IDeAUMsnAFzIgCuAtgEbQDcpyDV6A9sACasBfYsV4QEAGzhQUMGiARhgXEMl7AAznjhgEACwA8AaWQQAHpBC91yANYQAnlxhpMOfAD4AFGHt4IVQwAaZDApAHMIMCoMLDBcPABtQwBdAEpOHn4RDS0dXU8AcjgC4IBGACYAZlTWNU1tPUKGAprkAHo2tFM-BQheZHLyKDD6WPVghhowZDCuadKAOk9ygFYVgBZU5ABBFXDR8GQYLihkAtDhyILkAHc4axA55DwoLnQ+PuEfP2QAVRAlCAACpcACS4Gg6jEimU+l+7mQAF42J5fiZzFgrOQQPZkAB+ZCeUxUX5bREI7h8ZBUEAQTBQLZmCxYzxEqigGDQZCgskUzJbAmg6nUOnQYjfFAAERyDQMxkRtgcThcsXiCIV-0BIPBkCgUIUgKM6OZ1jsjmcMTcBAJ3l8-mQQRC4Ui0VccXwSTSSL5VJpoqg7lqYkk0mQCGU6mmdVyenKVGl9TytRleXKhWKZSqNWyidjTRazHanS4NmIQA

💻 Code

interface EventMap {
  a: number;
  b: void;
}

declare function dispatch<K extends keyof EventMap>(type: K, target: EventMap[K]): void;
dispatch('a', 123);
dispatch('b'); // Expected 2 arguments, but got 1.(2554) An argument for 'target' was not provided

type UnionToIntersection<U> =
  (U extends any ? (x: U) => void : never) extends ((x: infer I) => void) ? I : never
type Dispatch<K = keyof EventMap> = UnionToIntersection<K extends keyof EventMap ? (type: K, target: EventMap[K]) => void : never>;
declare const dispatch2: Dispatch;
dispatch2('a', 123);
dispatch2('b'); // ok

🙁 Actual behavior

Errors in code
Expected 2 arguments, but got 1.

🙂 Expected behavior

No Errors

Additional information about the issue

dispatch('b') matches the signature function dispatch<"b">(type: "b", target: void): void but still raises an error.

I tried use overloads as a work around, and that just works. So is this is a bug or intentional？

[jcalz]

duplicate #29131