pred_switch manual example

[Andy Balba]

The following example taken from 
https://dlang.org/library/std/algorithm/comparison/pred_switch.html

doesn't compile under gdc...Is there a way to fix it ?


int factorial(int n)
{
     return n.predSwitch!"a <= b"(
         -1, {throw new Exception("Can not calculate n! for n < 
0");}(),
         0, 1, // 0! = 1
         n * factorial(n - 1) // n! = n * (n - 1)! for n >= 0
         );
}
writeln(factorial(3));

The gdc compiler error is
  error: no property 'predSwitch' for type 'int'
      return n.predSwitch!"a <= b"(