[func.wrap.ref.deduct]

Constraints: is_function_v<F> is true.

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

Remarks: The deduced type is function_ref<F>.

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

Constraints:

(6.1)
F is of the form R(G::*)(A..) cv & ${o p t noexcept (E) for a type G, or}_{o p t noexcept (E) for a type G, or}$
(6.2)
F is of the form M G::* for a type G and an object type M, in which case let R be invoke_result_t<F, T&>, A.. be an empty pack, and E be true, or
(6.3)
F is of the form R(*)(G, A..) noexcept(E) for a type G.

Remarks: The deduced type is function_ref<R(A..) noexcept(E)>.

22 General utilities library [utilities]