5.6 regression: Incorrect param type inference for type with all optional props

🔎 Search Terms

inference, param

🕗 Version & Regression Information

• This changed between versions 5.5.4 and 5.6.0

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.0-dev.20240816#code/JYOwLgpgTgZghgYwgAgMoHsC2EAqBPABxQG8AoZC5AeiuQHkRkBWAOgDYWAGAGmQgA8iUYNnDIARnmSY4Aa1ABzZGAAWwAM7ICUdAWRQIARwCuwAwBMW5SsZDATEAAo6CAfgBcydWGEgFAblIAX1JScwgEABs4A2QEdBBvZHRxdU86VOgANzhxSIgAHgxsfCIAPkDSGFsEMGAE5BiFAHEIMHUAdR0-UogACgBKdMyoHLzC4iCy5DJKfTbjKEYU9RYCYCI+sDgCPqbkAF5p2bnKJsDTyhoAPVdrS5pkAFFBCMhzT2LcQhRQZhZWAAWe6nR4AQVqxjgkU8FEmyD+WVYHE4AFpxG04Mp0P8UajwlkWAAmThEwGcAAcAEY2Mg9mBkPk4N4BiCggMBoEQlUanUGk1Wu0AMLoKAGWq9QYzEEGMCLZapNYbfrbXb7I7Sy4Uc4guY3O5a6i0MGRADucDwmniYrebI5XNCoEgsEQKAy6myuXyBRwxxB6yIBTBZT6unSQjgYFFADFefUQD7eMGhvQRmNvcGHU7oPAkMgAKogGJ4WMgWrxxPIABKxzp6nQiyQnhwKar-mQIWzLrzdAjUagpfLCUrNb4-EgIHMmkLxcHfIT7s94x9ZV4i9GXsKNdrnfAOddyAAsgl0L1e9BIzG48PfWOJ1P6H2r2X55Xb8QO2EItFYtUX-HlB2Fc+hAAQwE8PockiYwIGbAZDmmLJ0GAcwU2PEBTx+c8oEvAdrwTX1AiAA

💻 Code

interface SomeType {
    // On 5.6.0, experiment by making this prop required.
    uniqueProp?: string;
}

declare const obs: Observable<SomeType>;

function argGetsWrongType(): Observable<{}> {
    return obs.pipe(tap(arg => {
        arg;
        //^?
        // Expected: SomeType in 5.5.4
        // Actual:   {} in v5.6.0-beta to 5.6.0-dev.20240816 (at least)
    }));
}

function argGetsCorrectType() {
    return obs.pipe(tap(arg => {
        arg;
        //^?
        // Always correct
    }));
}

interface Observable<T> {
    pipe<A>(op: OperatorFunction<T, A>): Observable<A>;
}
interface UnaryFunction<T, R> { (source: T): R; }
interface OperatorFunction<T, R> extends UnaryFunction<Observable<T>, Observable<R>> { }
interface MonoTypeOperatorFunction<T> extends OperatorFunction<T, T> { }
declare function tap<T>(next: (value: T) => void): MonoTypeOperatorFunction<T>;

🙁 Actual behavior

arg is inferred as {}

🙂 Expected behavior

arg is inferred as SomeType as was consistent in v5.5.4

Additional information about the issue

(Google note: See http://cl/664889390 for local workaround)

Bắt đầu với TypeScript Playground được liên kết và so sánh việc tái hiện giữa 5.5.4 và 5.6.0. Theo dõi suy luận generic liên quan đến hai lời gọi pipe/tap, sau đó xác minh rằng tham số callback được suy luận là SomeType khi tất cả các thuộc tính của nó đều là tùy chọn, trong khi trường hợp đối chứng hiện có vẫn chính xác.