$ cat a.d
import b;
void fa() {
        fb!int();
}

$ cat b.d
struct S(T) {
        void fs() {
                fb!int();
        }
}
S!T fb(T)() {
        return S!T();
}

$ dmd -c -o- a b

$ dmd -c -o- b a
b.d(3): Error: forward reference to fb
b.d(6): Error: template instance b.S!(int) error instantiating
a.d(3):        instantiated from here: fb!(int)
a.d(3): Error: template instance b.fb!(int) error instantiating