struct Delegate1(T) {}
struct Delegate2(T1, T2) {}

template Signature(T)
{
  alias Signature = typeof(*(T.init));
}

alias del1_t = Delegate1!(Signature!(void function()));
alias del2_t = Delegate2!(Signature!(int function(float, double)), Signature!(int function(float, double)));

extern(C++) 
{
  void DelegateTest1(del1_t);
  void DelegateTest2(del2_t);
}

When compiling with the latest dmd from git head on windows this will output:
Error: ICE: Unsupported type void()
Error: ICE: Unsupported type int(float, double)
Error: ICE: Unsupported type int(float, double)

It should mangle the functions as:
static assert(DelegateTest1.mangleof == "?DelegateTest1@@YAXU?$Delegate1@$$A6AXXZ@@@Z");
static assert(DelegateTest2.mangleof == "?DelegateTest2@@YAXU?$Delegate2@$$A6AHMN@Z$$A6AHMN@Z@@@Z");