Purity not enforced for default arguments?

[Xinok via Digitalmars-d-learn]

The following code fails to compile because unpredictableSeed is 
impure:

     void main()
     {
         foreach(i; 0..10) writeln(pureRandom);
     }

     pure uint pureRandom()
     {
         auto r = Random(unpredictableSeed);
         return r.front;
     }

However, make unpredictableSeed a default argument, wrap the call 
in another pure function, and it compiles:

     void main()
     {
         foreach(i; 0..10) writeln(pureRandom2);
     }

     pure uint pureRandom2()
     {
         return pureRandom;
     }

     pure uint pureRandom(uint seed = unpredictableSeed)
     {
         auto r = Random(seed);
         return r.front;
     }

I'm inclined to believe this is a bug. While pureRandom could be 
considered weakly pure, pureRandom2 has no arguments so it should 
be strongly pure. Yet, it yields a different value on every call.