Problem with casting instance reference to void* and back.

[Vlad Leberstein via Digitalmars-d-learn]

Hi! My use case requires interaction with C API which in turn 
implies storing object instance reference as void *. I'm using 
gdc 4.9.2 and everything worked fine with "object -> void * -> 
object" conversion, but "object -> void * -> interface" failed. 
The stripped-down example is something like this:

interface TestInterface {
     void testMethod();
}

class TestImpl : TestInterface {
     void testMethod() {
         writefln("TestImpl::testMethod\n");
     }
};

void testDispathcer(void *rawSelf) {
     auto self = cast(TestInterface) rawSelf;
     // nothing happens
     self.testMethod();
}


int main(string[] args) {
     auto t = new TestImpl();
     testDispathcer(cast(void *) t);
     return 0;
}

However this works:

void testDispathcer(TestImpl rawSelf) {
     auto self = cast(TestInterface) rawSelf;
     // TestImpl::testMethod is printed to stdout
     self.testMethod();
}


int main(string[] args) {
     auto t = new TestImpl();
     testDispathcer(t);
     return 0;
}

Is there any way to handle such situation without resorting to 
templating dispatcher with every *Impl type? As second example 
works as expected I presume that runtime loses some type 
information to perform downcast and maybe it's possible to 
extract it and store externally and merge back later?

I'm quite new to D and sorry if the question is dumb. Any help 
would be greatly appreciated! Thanks in advance!