Declared in <absl/functional/function_ref.h>
template<
auto F,
typename Obj,
typename = EnableIfCompatible<decltype(F), std::remove_reference_t<Obj>&>,
absl::functional_internal::EnableIf<! std::is_rvalue_reference_v<Obj &&>> = 0>
FunctionRef(
nontype_t<F> arg,
Obj&& obj) noexcept;