int main() {
    B bb;                           //does not compile (neither does class B bb;)
    C cc;                           //does not compile

    struct t tt;                    //compiles

    class B {};
    struct s { struct t * pt; };    //compiles
    struct t { struct s * ps; };

    return 0;
}

class C {};

I just modified the example given here.

Why is that the struct forward declarations work but not the class forward declarations?

Does it have something to do with the namespaces - tag namespace and typedef namespace? I know that the structure definitions without typedefs go to tag namespace.

Structures are just classes with all public members. So, I expect them to behave similarly.