As per my comment:
sizeof((x)[0]) will give an error if the type is not an "indexable" type - however, if it happens to be a pointer, it will happily take that. And also, if there is an operator[] for the type of x.
It is quite hard in C to do this, but C++ may allow some template type solutions (I don't actually know how to do that, as I've never tried to do that, or anything similar with templates).
