GCC gained a new warning, -Wimplicit-fallthrough, which warns about
implicitly falling through a case statement. The regular expressions
used at the default level (-Wimplicit-fallthrough=3) don't match with
a colon at the end. The comment also needs to be followed (after
optional whitespace and other comments) by a 'case' or 'default'
keyword, i.e. it will not be recognized with a '}' between the comment
and the keyword.