How to port C++ std::is_reference to D ?

[wjoe via Digitalmars-d-learn]

Hello,

I'm choking on a piece of C++ I have no idea about how to 
translate to D.

  template <typename T,
        typename std::enable_if< std::is_const<T>::value == true, 
void>::type* = nullptr>
    constexpr const char *modifier() const {
        return "[in] ";
    }

  template <typename T,
        typename std::enable_if< std::is_reference<T>::value == 
true, void>::type* = nullptr>
    constexpr const char *modifier() const {
        return "[out] ";
    }

my attempt at it is like this:

  template modifier(T) {

      static if (is (T==const)) {

          const char* modifier = "[in] ";

      } else static if (/* T is a reference ?*/) { // [*]

          const char* modifier = "[out] ";
      }
  }

but even if I could e.g. say something like
  is(T == ref R, R),
  auto a = modifier!(ref T);
wouldn't work.