There is no test for interface-to-class casts in the D/Objective-C test suite, which means you're likely the first person to try that. It's probably just an oversight in the compiler code.
Hey Michel, thanks very much for this explanation! That's actually good news. It certainly will be good to have this fixed, as the target action mechanism gets a lot of it's spice that the "sender" only needs to comply to a protocol and can be whatever control or even nil.