interp: case ignores the ;;& and ;& fallthrough terminators #1391

case parses bash's ;;& and ;& fallthrough terminators and then treats both as plain ;;.

$ gosh -c 'case ab in a*) echo one;;& *b) echo two;; esac'
one
$ bash -c 'case ab in a*) echo one;;& *b) echo two;; esac'
one
two

$ gosh -c 'case ab in a*) echo one;& *) echo two;; esac'
one
$ bash -c 'case ab in a*) echo one;& *) echo two;; esac'
one
two

Both parse without error, so the information is present in the AST and dropped at execution.

;;& is how a case classifies along several axes at once, e.g.

case "$file" in
  *.tar*)  untar=1 ;;&
  *.gz)    decomp=gunzip ;;
  *.bz2)   decomp=bunzip2 ;;
esac

For *.tar.gz this sets untar=1 and never selects a decompressor, so the script proceeds with an unset variable and the symptom appears elsewhere.

A silently-ignored terminator is worse than a parse error, which would at least stop on the line responsible. If honoring them is not desirable, rejecting them at parse time would be strictly better than the current behavior.

Found by a bash-vs-gosh differential sweep (128 snippets, each run direct and through eval) while integrating interp as the substrate of a shell. Reproduced on gosh@v3.13.2-0.20260813221317-a04df9f0d4b8 against bash 5.3.15 on darwin/arm64. Happy to send a PR if the direction looks right.