interp: $- is unset, so it is empty and errors under set -u #1398

$-, the POSIX special parameter holding the current option flags, is never set.

$ gosh -c 'echo "[$-]"'
[]
$ bash -c 'echo "[$-]"'
[hBc]

$ gosh -c 'set -e; echo "[$-]"'
[]
$ bash -c 'set -e; echo "[$-]"'
[ehBc]

Being empty rather than absent would already be a divergence, but it is genuinely unset, so set -u turns any reference into a fatal error:

$ gosh -c 'set -u; echo "[$-]"'
-: unbound variable
$ bash -c 'set -u; echo "[$-]"'
[uhBc]

That combination is the awkward one: set -euo pipefail followed by anything that consults $- aborts. The common idioms are interactivity checks in shell rc files and libraries that save and restore options:

case $- in *i*) ... ;; esac        # sees no flags at all
[[ $- == *e* ]] && restore_errexit=1

The interactivity check happens to reach the right answer today, since an empty $- contains no i either — but only by accident, and only while interp has no interactive mode.

$- should hold the single-letter names of the enabled options, which interp already tracks in posixOptsTable; note that pipefail has no letter there ( ), so it is -o-only in bash too and should be left out.

Reproduced on master at d6550df against bash 5.3.15 on darwin/arm64, found while measuring set for #1392.