declval

I'm trying to understand the difference between declval<T>() and declval<T&>()? Is there an example where T& can be used while T cannot?

#include <type_traits>
#include <utility>

struct X {
  X() = delete;
  int func();
};

int main()
{
// works with both X as well as X& within declval
  static_assert(std::is_same_v<decltype(std::declval<X&>().func()), int>);
}