TypeScript unable to infer types properly between interfaces and overloaded methods. #43643
Description
Bug Report
In all versions of TS I can find, there's no way to mark certain call patterns of a method as deprecated while maintaining inference with certain interfaces. If an interface is split out into different overloads so certain overloads can be deprecated, then it breaks inference in some cases.
🔎 Search Terms
I'm not sure what to search for here. Interfaces?
🕗 Version & Regression Information
- All versions
⏯ Playground Link
Playground link with relevant code
💻 Code
interface Observer<T> {
next(value: T): void;
error(error: any): void;
complete(): void;
}
interface Unsubscribable {
unsubscribe(): void;
}
interface Subscribable<T> {
subscribe(observer?: Partial<Observer<T>>): Unsubscribable;
}
class Observable<T> implements Subscribable<T> {
/////////////////////////////////////////
// Valid call pattern
/////////////////////////////////////////
subscribe(observerOrNext?: Partial<Observer<T>> | ((value: T) => void)): Unsubscribable;
//////////////////////////////////////////
// Deprecated call patterns
//////////////////////////////////////////
/**
* @deprecated This call pattern is going away
*/
subscribe(next: null | undefined, error: null | undefined, complete: () => void): Unsubscribable;
/**
* @deprecated This call pattern is going away
*/
subscribe(next: null | undefined, error: (error: any) => void, complete?: () => void): Unsubscribable;
/**
* @deprecated This call pattern is going away
*/
subscribe(next: (value: T) => void, error: null | undefined, complete: () => void): Unsubscribable;
/**
* @deprecated This call pattern is going away
*/
subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): Unsubscribable;
////////////////////////////////////////////
// Impl
////////////////////////////////////////////
subscribe(...args: any[]): Unsubscribable {
return {
unsubscribe() {}
};
}
}
function somethingThatTakesSubscribable<T>(arg: Subscribable<T>): T {
return null!;
}
/////////////////////////////////////
// Our failed expectation here
// (it's unknown)
/////////////////////////////////////
const result = somethingThatTakesSubscribable(new Observable<number>()); // $ExpectType number🙁 Actual behavior
result is unknown above.
🙂 Expected behavior
result would be number.