Casting to interface not allowed in @safe code?

[Jim]

Hi,

consider this:

interface Base
{
   void setup();
}

interface FeatureX
{
   void x();
}

class Foo: Base, FeatureX
{
   void setup(){};
   void x(){};
}

void main()
{
   Base foo = new Foo(); // This would be the result of a factory 
class

   (cast(FeatureX)foo).x(); // 1
}

1) error: casting to interface FeatureX is not allowed in @safe 
code.

Question: How to call foo.x in @safe code ?