is(T : long) vs is(T == long)

[Caligo]

What is the difference between this:
template isNumerik(T){
  enum bool isNumerik = is(T : long) || is(T : real);
}

and this:

template isNumerik(T){
  enum bool isNumerik = is(T == long) || is(T == real);
}


They both work and I can't find anywhere in the book where it talks about the :