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Implement the Stage 3 Decorators Proposal #50820

Improve checking of in operator #50666

Type inference in conditional types #21496

Conditional types #21316

下面的示例来自 Improve checking of in operator #50666

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const sym = Symbol();

function f1(x: unknown) {
    if (x && typeof x === "object" && "a" in x && 1 in x && sym in x) {
        x;  // Record<"a", unknown> & Record<1, unknown> & Record<typeof sym, unknown>
        x.a;  // Ok
        x[1];  // Ok
        x[sym];  // Ok
    }
}

function f2(x: { a: string } | { b: string }) {
    if ("a" in x) {
        x;  // { a: string }
    }
    else if ("b" in x) {
        x;  // { b: string }
    }
    else {
        x;  // never
    }
}

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