[Bug libstdc++/66284] std::reference_wrapper is transparent to std::function::target_type

[potswa at mac dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66284

--- Comment #2 from David Krauss <potswa at mac dot com> ---
The converting constructor requirements also say more explicitly,

  shall not throw exceptions when f is a function pointer
  or a reference_wrapper<T> for some T.

Probably the copy constructor should be worded similarly.

In any case, the exception guarantee isn't affected, as
sizeof(reference_wrapper<T>) == sizeof(T*).