bug in 'set -n' processing

[Eric Blake]

Bash has a bug: ${+} is syntactically invalid, as evidenced by the error
message when running the script, yet using 'set -n' was not able to flag
it as an error.

$ echo $BASH_VERSION
4.2.8(1)-release
$ bash -c 'echo ${+}'; echo $?
bash: ${+}: bad substitution
1
$ bash -cn '${+}'; echo $?
0
$ ksh -cn '${+}'; echo $?
ksh: syntax error at line 1: `+' unexpected
3

Meanwhile, a feature request: since $+ outputs a literal "$+", it is
proof that + cannot be a valid variable name.  Bash should follow ksh'
lead by having 'set -n' warn about suspicious but usable constructs, at
least when --posix is not in effect.

$ bash -c 'echo $+'
$+
$ bash -cn '$+'; echo $?
0
$ ksh -cn '$+'; echo $?
ksh: warning: line 1: $ not preceded by \
0

-- 
Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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