type Func = <T extends ["a", number] | ["b", string]>(...args: T) => void;

const f1: Func = (kind, payload) => {
    if (kind === "a") {
        payload.toFixed();  // error
    }
    if (kind === "b") {
        payload.toUpperCase();  // error
    }
};

type Func = (...args: ["a", number] | ["b", string]) => void;

const f1: Func = (kind, payload) => {
    if (kind === "a") {
        payload.toFixed();  // 'payload' narrowed to 'number'
    }
    if (kind === "b") {
        payload.toUpperCase();  // 'payload' narrowed to 'string'
    }
};

f1("a", 42);
f1("b", "hello");