typeid of variable of interface type holding reference to derived class

[kdevel via Digitalmars-d-learn]

I would expect the unittest below to pass. Could not find
any explanation in [1].

~~~typeid.d
class A {}
class B : A {}
class C : B {}

interface Q {}
class R : Q {}
class S : R {}

unittest {
   import std.stdio : writeln;

   void runtest (X, Y) ()
   {
      X x = new Y ();
      writeln ("X = ", typeid (X), ", Y = ", typeid (Y));
      assert (typeid (x) == typeid (Y));
   }

   runtest!(A, C); // pass
   runtest!(R, S); // pass
   runtest!(Q, S); // fail
   runtest!(Q, R); // would also fail
}
~~~

$ dmd -checkaction=context -unittest -main -run typeid
X = typeid.A, Y = typeid.C
X = typeid.R, Y = typeid.S
X = typeid.Q, Y = typeid.S
typeid.d(16): [unittest] typeid.Q != typeid.S
1/1 unittests FAILED

[1] https://dlang.org/spec/interface.html