IFS=''			# testing with only empty IFS

set -- this is a test

printf '|%s|\n' ${1+"$@"}
echo
printf '|%s|\n' "${1+$@}"
echo
printf '|%s|\n' "$@"
echo

printf '|%s|\n' ${1-"$@"}
printf '|%s|\n' "${1-$@}"

echo
: ${foo:="$@"}
printf '|%s|\n' "$foo"

unset foo
: "${foo:=$@}"
printf '|%s|\n' "$foo"

unset foo
printf '|%s|\n' ${foo-"$@"}
printf '|%s|\n' "${foo-$@}"