I'm not super familiar with Cello (and I'm not sure I get the point of it overall, are there that many platforms left that you can target from C but not C++?) but in its defense it does seem to implement fat pointers and runtime checks to have a degree of type safety. Not sure how thorough it is but it's not just decaying everything to void pointers behind the scenes.

It's a pretty clever hack though, like using setjmp for exception handling. I'm pretty sure I'd never want to use that in production anywhere but it was probably fun to implement.