[generics] Can not find an exists chunk in the heap.

[necto]

I use the exists<t>(_) mechanism to pass the type argument to a generic function. However I also need a t -> int fixpoint function which I also pass via the exists<fixpoint(t,int)> chunk.
To avoid confusion between the two, I merged them into exists<pair<t, fixpoint(t,int)> > and pass both a dummy t value and a function.
VeriFast fails to find this chunk, albeit it is visible in the heap panel in vfide.
Here is the code:

int calee/*@ <t> @*/()
//@ requires exists<pair<t, fixpoint(t,int)> >(pair(_, ?hsh));
//@ ensures true;
{
  return 0;
}

//@ fixpoint int idi(int x) { return x; }

int caller()
//@ requires true;
//@ ensures true;
{
  //@ close exists(pair(0, idi));
  calee();
}

[necto]

The workaround here is to define a custom predicate, like:

//@ predicate type_arg<t>(t x, fixpoint(t,int) f) = true;

And use it instead of exists this seems to work.