function isFunction<T>(value: T): value is Extract<T, Function> {
    return typeof value === "function";
}

// Return type is Extract<T, Function>, i.e. a T known to be a Function
function getFunction<T>(item: T) {
    if (isFunction(item)) {
        return item;
    }
    throw new Error();
}

function f1<T>(x: T) {
    if (isFunction(x)) {
        const f: Function = x;  // x is a Function
        const t: T = x;  // and x is a T
    }
}

function f2(x: string | (() => string) | undefined) {
    if (isFunction(x)) {
        x();  // x narrowed to () => string
    }
}

function f3(x: string | (() => string) | undefined) {
    const f = getFunction(x);  // () => string
    f();
}