The following code causes forward reference:

template Mixin ()
{
	import std.stdio; // it doesn't matter what import it is
	int i;
}

class Bar : Foo
{
	mixin Mixin;
}

class Foo
{
	mixin Mixin;
}

Flip the class declarations to get the code to compile. The problem is harder to work around with the classes in individual modules.