Purpose of perfect forwarding for Callable argument in invocation expression?

问题

In Scott Meyer's book Effective Modern C++ on page 167 (of the print version), he gives the following example:

auto timeFuncInvocation = [](auto&& func, auto&&... params) {
  // start timer;
  std::forward<decltype(func)>(func)(
    std::forward<decltype(params)>(params)...
  );
  // stop timer and record elapsed time;
};

I completely understand the perfect forwarding of params, but it is unclear to me when perfect forwarding of func would ever be relevant. In other words, what are the advantages of the above over the following:

auto timeFuncInvocation = [](auto&& func, auto&&... params) {
  // start timer;
  func(
    std::forward<decltype(params)>(params)...
  );
  // stop timer and record elapsed time;
};

回答1:

For the same purpose as for arguments: so when Func::operator() is a ref-qualified:

struct Functor
{
    void operator ()() const &  { std::cout << "lvalue functor\n"; }
    void operator ()() const && { std::cout << "rvalue functor\n"; }
};

Demo

来源：https://stackoverflow.com/questions/36920485/purpose-of-perfect-forwarding-for-callable-argument-in-invocation-expression