[libc++] std::transform with an identity predicate is much slower than std::copy

[ldionne]

This needs to be investigated.

[denzor200]

Having looked at godbolt snippet, this is not only libc++ issue. Suppose the simpliest way is to add std::transform's overload that receives std::identity: https://godbolt.org/z/8bjcMbqzn

[winner245]

We may add an overload for identity predicate:

template <class _InputIterator,
          class _OutputIterator,
          class _UnaryOperation,
          __enable_if_t<__is_identity<_UnaryOperation>::value, int> = 0>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator
transform(_InputIterator __first, _InputIterator __last, _OutputIterator __result, _UnaryOperation) {
  return std::copy(__first, __last, __result);
}

But then I realized that this only works with std::identity and its reference_wrappers. To make it more useful, we need a way to identify identity lambdas [](auto x) { return x; } or user-defined identity functions. Otherwise, this approach seems not very useful.