function default parameters lost

[Paul D Anderson via Digitalmars-d-learn]

I'm trying to pass a function pointer while keeping the default 
parameter values intact. Given the following:

import std.traits;
import std.stdio;

int foo(int a, int b = 1)
{
   return a;
}

alias FOOP = int function(int, int = 1);

struct ST(POOF)
{
   FOOP fctn;

   this(POOF fctn)
   {
     this.fctn = fctn;
   }

   void details()
   {
     alias PDVA = ParameterDefaultValueTuple!fctn;
     writefln("typeid(PDVA[0]) = %s", typeid(PDVA[0]));
     writefln("typeid(PDVA[1]) = %s", typeid(PDVA[1]));
   }
}

void main()
{
   FOOP fp = &foo;
   auto st = ST!FOOP(fp);
   st.details;
}

The default parameter value types are void, int: a has no default 
and b has an int value as its default.

If I change line 14 from
   FOOP fctn;
to
   POOF fctn;

The default parameter value types are void, void. In other words 
the default value for b is no longer there.

Why doesn't invoking the template (ST!FOOP) replace POOF in line 
14 with FOOP?

Paul