In the class synopsis:

  template<auto f>
    function_ref(nontype_t<f>, auto) -> function_ref<see below>;

The detailed specifications uses a forwarding parameter.
That's consistent with the actual constructors,
as it doesn't require copy on the caller side when deducing.

    function_ref(nontype_t<f>, auto&&) -> function_ref<@\seebelow@>;

In the deduction guide's detailed specifications:

template<auto f, class T>
  function_ref(nontype_t<f>, T&&) -> function_ref<see below>;

The second parameter in the class synopsis is actually auto.
That requires a copy, as opposed to T&&.
So I can't solve this editorially here,
but it can be done in the class synopsis.

To be consistent with my suggestion on the class synopsis:

template<auto f>
  function_ref(nontype_t<f>, auto&&) -> function_ref<@\seebelow@>;

Originally: #6357 (comment), #6357 (comment).