SFINAE with ampersand before a function vs its name

The code below correctly checks if the type T has a method sort. But when I modify the line marked (*) by changing decltype(&U::sort,...) to decltype(U::sort,...) (the symbol & is removed), then the code returns always false.

Why?

Why the name itself is not enough? What does this & mean?

#include <iostream>
#include <type_traits>

template <typename T>
class has_sort {

  template <typename U>
  static auto check(bool) -> decltype(&U::sort, std::true_type()); // (*)

  template <typename U>
  static std::false_type check(...);

public:
  using type = decltype(check<T>(true));
  static bool const value = type::value;
};

int main() {
  struct Foo { void sort(); };
  struct Foo2 { void sort2(); };
  std::cout << "Foo: " << has_sort<Foo>::value << std::endl;
  std::cout << "Foo2: " << has_sort<Foo2>::value << std::endl;
  std::cout << "int: " << has_sort<int>::value << std::endl;
}