As a superstitious C programmer, typedefing (void star) feels like walking on the cracks in the pavement, crossing the path of a black cat, walking under a ladder, or squeezing a lemon under the full moon to me. These kinds of tricks seem very clever at first but there always comes a point when they start to break down. I'd be leery about using union in 2017, but typedefing (void star) is like putting on your underpants outside your trousers, thinking you're superman and jumping out of a window thinking that you can fly.
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.
