I suspect there are multiple reasons for which the code below does not compile. It should, however, compile and run properly:

auto max(T...)(T a) 
      if (T.length == 2 
         && is(typeof(a[1] > a[0] ? a[1] : a[0]))
       || T.length > 2 
         && is(typeof(max(max(a[0], a[1]), a[2 .. $])))) {
   static if (T.length == 2) {
      return a[1] > a[0] ? a[1] : a[0];
   } else {
      return max(max(a[0], a[1]), a[2 .. $]);
   }
}

void main() {
    assert(max(4, 5) == 5);
    assert(max(3, 4, 5) == 5);
}