Spreading Parameters of ternary function type does not recognize parameter type

[alanthai]

TypeScript Version: 4.0.2

Search Terms: spread, parameters, ternary

Code

type EventCallback<T extends EventType> = T extends 'fetch'
  ? (meta: {url: string; duration: number}) => void
  : never;

type EventType = 'fetch';

const listeners = new Map<string, EventCallback<any>[]>();

const getListeners = (eventType: EventType) => listeners.get(eventType) || [];

export function dispatch<T extends EventType>(
  eventType: T,
  args: Parameters<EventCallback<T>>,
) {
  getListeners(eventType).forEach((listener) => listener(...args));
}

Expected behavior:
No error

Actual behavior:

Playground Link: https://www.typescriptlang.org/play?#code/C4TwDgpgBAogbhAdsAwgQwDYYEZoMYDWAPACpQQAewSAJgM6wLIngQB8UAvFGZdYvSgByAGYRgeABZCAUFCgB+KAAoAtuLQAuKAG8ArgCcM2usAMBLRAHMA3FBqG0wcwHtE2xHtXYIBgL4AlFwccC7mNHJQHhAIBjYyMqCQjEjALMncouJSQvEyeG6mUBjmpki+DNyIEADuUACyaGBEphbWADQpyOhYuIREaIggbADaALpsygF5BYhFVuIAMqX8FVwqManpENrwW6xBnBwlZdUGdAB0C8DKm8wHUAA+j1DjeZRgLgbAUCJ6iHhnG57KUwE4pKRyFRaAw9vdIJNInc0qxtCR2pE0AYrHRtAAFLFodTUc5EOGoTA4fDEEhsNgYoI6SLXZanCq3JgoyABC4iL4wfCSZTKE6rAyHY4rcoGZQXOVYnEBaYyPwyIA

Related Issues: None

[ahejlsberg]

This is basically a design limitation. A spread argument ...args is processed as synthetic discrete arguments only when the type of args is an actual tuple type. In this particular case, args is of a type that is assignable to a tuple type, but it isn't an actual tuple type. For that reason, arity checking assumes that the spread argument could be of any length, including zero, and therefore the arity check fails. Unfortunately, it would be non-trivial to do better here.