using GenerateTimestampsFn = function_ref<Timestamp(uint64_t nticks)>;

void doSomethingOnVector(const std::vector<BSONObj>& entries, GenerateTimestampsFn timestampFn) {
    ...
    timestampFn(entries.size());
}

// Call site
void main() {
    // This line is problematic! 'f' is a function_ref and the object it's pointing
    // to will be destroyed.
    GenerateTimestampsFn f = [&](uint64_t nticks) {
        return Timestamp(100, 1);
    };

    doSomethingOnVector(entries, f);
}

void main() {
    // Using `auto` now.
    auto f = [&](uint64_t nticks) {
        return Timestamp(100, 1);
    };

    doSomethingOnVector(entries, f);
}