Is instantiabilty of templated types decidable?

[Manfred Nowak]

This code implements sort of an unary counter: 

class Elem( size_t mynumber) {
  Elem!( mynumber +1)* next;
}
void main(){
  auto unaryCounter= new Elem!0;
}

Although only `Elem!0' has to be instantiated dmd 2.060 on 
windows shouts:
  Error: template instance Elem!(500u) recursive expansion

This is the reason for the subject. Because:

a) Instantiability is decidable
Why does the compiler stop with the evaluation at that randomly 
choosen and apparently hard coded value of 500 recursive 
expansions?

b) Instantiability is not decidable
Why does the compiler even try to instantiate more than the 
indeed needed type `Elem!0'?

I miss the rationale for this behaviour.

-manfred