constexpr function_ref& operator=( const function_ref& ) noexcept = default;

template< class T > 
constexpr function_ref& operator=( T ) = delete;

1) Copy assignment operator is explicitly-defaulted. std::function_ref satisfies copyable and TriviallyCopyable. This defaulted assignment operator performs a shallow copy of the stored thunk-ptr and bound-entity.

2) User-defined assignment operator is explicitly-deleted if T is not the same type as std::function_ref, std::is_pointer_v<T> is false, and T is not a specialization of std::nontype_t. This overload participates in overload resolution only if the constraints are satisfied in the conditions above.