Test case:

struct RefCounted(T)
{
    struct RefCountedStore
    {
        private struct Impl
        {
            T _payload;
        }

        private Impl* _store;
    }
    RefCountedStore _refCounted;

    ~this()
    {
        import core.stdc.stdlib : free;
    }
}

struct GroupBy(R)
{
    struct SharedInput
    {
        Group unused;
    }

    struct Group
    {
        private RefCounted!SharedInput _allGroups;
    }
}

void main()
{
    GroupBy!(int[]) g1;
}

Output:

test.d(5): Error: struct test.RefCounted!(SharedInput).RefCounted.RefCountedStore.Impl unable to resolve forward reference in definition
test.d(29): Error: template instance test.RefCounted!(SharedInput) error instantiating
test.d(35):        instantiated from here: GroupBy!(int[])
test.d(35): Error: template instance test.GroupBy!(int[]) error instantiating