# First try some invalid patterns. Do in subshell due to parsing error.
# (set argv0 to "SHELL" to avoid "/path/to/shell: blah" in error messages)
"$THIS_SH" -c 'echo ${+}  ; echo moo' SHELL
"$THIS_SH" -c 'echo ${:+} ; echo moo' SHELL

# now some funky ones.
# ${V+word} "if V unset, then substitute nothing, else substitute word"
# ${V:+word} "if V unset or '', then substitute nothing, else substitute word"
#
# ${#:+} is a :+ op on $#, but ${#+} (and any other ${#c}) is "length of $c",
# not + op on $#.
# bash and dash do not accept ${#+}. it's possible for some shell to skip
# the check that c is valid and interpret ${#+} as "len of $+". Not testing it.
# echo _${#+}_
echo _${#:+}_
# Forms with non-empty word work as expected in both ash and bash.
echo _${#+z}_ _${#:+z}_

# now some valid ones
set --
echo _$1 _${1+} _${1:+} _${1+word} _${1:+word}

set -- aaaa
echo _$1 _${1+} _${1:+} _${1+word} _${1:+word}

unset f
echo _$f _${f+} _${f:+} _${f+word} _${f:+word}

f=
echo _$f _${f+} _${f:+} _${f+word} _${f:+word}

f=fff
echo _$f _${f+} _${f:+} _${f+word} _${f:+word}