Ugh, very simply? Any STATEMENT is allowed to be, among other things, a LABELED_STATEMENT which is (<ident> ":" STATEMENT | "case" <constexpr> ":" STATEMENT | "

Ugh, very simply? Any STATEMENT is allowed to be, among other things, a LABELED_STATEMENT which is (<ident> ":" STATEMENT | "case" <constexpr> ":" STATEMENT | "default" ":" STATEMENT). Then in the semantic pass it's checked that labeled statements starting with case/default must be inside some switch. That's it.