That's the exact output of dmd 2.059head, on win 7:                 
Assertion failure: 't' on line 7911 in file 'mtype.c'

abnormal program termination

//code
struct A{
  int a;
}

template Inherit(alias X)
{
  X __super;
  alias __super this;
}

struct B{
  mixin Inherit!A;
  int b;
}

struct PolyPtr(X)
{
	X* _payload;
	static if(is(typeof(X.init.__super)))
	{
		alias typeof(X.init.__super) Super;
		@property auto getSuper(){ return PolyPtr!Super(&_payload.__super); }
		alias getSuper this;
	}
//	alias _payload this;//multiple alias this, sigh
	auto opDispatch(string s)(){ return mixin("_payload."~s); }
}

template create(X) 
{
	PolyPtr!X create(X, T...)(T args){
		return PolyPtr!X(args);
	}	
}

void f1(PolyPtr!A a) {/*...*/}
void f2(PolyPtr!B b) {f1(b);/*...*/}

void main(){
	auto b = create!B();
}