The error message for the second case isn't as clear as it could be, if you
don't know what to look for maybe. It says:
Error: type mismatch: got <set[range 0..65535(int)]> but expected 'CharSet
= set[int16]'
Run
See the (int) after the range ...? That tells you the type it gets is actually
of type int. Your set however takes int16. So to make it work you have to give
explicit int16 literals:
x = {1'i16..9'i16, 15'i16, 45'i16..78'i16}
Run
