Segfault while using zsh/param/private module

[Alezender Tan <tan.wei.xin.alezender+zsh@gmail.com>]

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Hello,

I have the following piece of code involving the use of the
zsh/param/private module that segfaults as seen below
```
➜  ~ zsh -f -x -c 'zmodload zsh/param/private
function _TEST {
  private arg
  for arg in "${@}"; do
    case "${arg}" in
      test1=*)
        private test1="${arg#*=}"
      ;;

      test2=*)
        private test2="${arg#*=}"
      ;;
    esac
 done

 echo "test1=${test1}"
 echo "test2=${test2}"
}

_TEST test1="" test2=""'
+zsh:1> zmodload zsh/param/private
+zsh:20> _TEST 'test1=' 'test2='
+_TEST:1> private arg
+_TEST:2> arg=test1=
+_TEST:3> case test1= (test1=*)
+_TEST:5> private 'test1='
+_TEST:2> arg=test2=
+_TEST:3> case test2= (test1=*)
+_TEST:3> case test2= (test2=*)
+_TEST:9> private 'test2='
[1]    3152586 segmentation fault (core dumped)  zsh -f -x -c
```

It's happening on the latest version of zsh which I installed using brew on
Ubuntu 20.04 (was using 5.8 installed via apt but uninstalled to check if
the issue was still occurring on 5.9)
```
➜  ~ zsh --version
zsh 5.9 (x86_64-pc-linux-gnu)
➜  ~ brew info zsh
==> zsh: stable 5.9 (bottled), HEAD
UNIX shell (command interpreter)
https://www.zsh.org/
/home/linuxbrew/.linuxbrew/Cellar/zsh/5.9 (1,578 files, 14.9MB) *
  Poured from bottle using the formulae.brew.sh API on 2023-07-19 at
22:38:08
From: https://github.com/Homebrew/homebrew-core/blob/HEAD/Formula/zsh.rb
License: MIT-Modern-Variant
==> Dependencies
Build: texinfo ✘
Required: ncurses ✔, pcre ✔, gcc ✔, glibc ✔
==> Options
--HEAD
        Install HEAD version
==> Analytics
install: 7,575 (30 days), 28,511 (90 days), 38,178 (365 days)
install-on-request: 7,409 (30 days), 27,934 (90 days), 37,383 (365 days)
build-error: 0 (30 days)
```

The segfault occurs regardless of whether `test1` and `test2` are empty or
non-empty strings as seen below
```
➜  ~ zsh -f -x -c 'zmodload zsh/param/private
function _TEST {
  private arg
  for arg in "${@}"; do
    case "${arg}" in
      test1=*)
        private test1="${arg#*=}"
      ;;

      test2=*)
        private test2="${arg#*=}"
      ;;
    esac
 done

 echo "test1=${test1}"
 echo "test2=${test2}"
}

_TEST test1="test3" test2="test4"'
+zsh:1> zmodload zsh/param/private
+zsh:20> _TEST 'test1=test3' 'test2=test4'
+_TEST:1> private arg
+_TEST:2> arg=test1=test3
+_TEST:3> case test1=test3 (test1=*)
+_TEST:5> private 'test1=test3'
+_TEST:2> arg=test2=test4
+_TEST:3> case test2=test4 (test1=*)
+_TEST:3> case test2=test4 (test2=*)
+_TEST:9> private 'test2=test4'
[1]    3155156 segmentation fault (core dumped)  zsh -f -x -c
```

But strangely enough, does not happen when `test2` is omitted from the code
altogether as seen below
```
➜  ~ zsh -f -x -c 'zmodload zsh/param/private
function _TEST {
  private arg
  for arg in "${@}"; do
    case "${arg}" in
      test1=*)
        private test1="${arg#*=}"
      ;;
    esac
 done

 echo "test1=${test1}"
}

_TEST test1=""'
+zsh:1> zmodload zsh/param/private
+zsh:15> _TEST 'test1='
+_TEST:1> private arg
+_TEST:2> arg=test1=
+_TEST:3> case test1= (test1=*)
+_TEST:5> private 'test1='
+_TEST:10> echo 'test1='
test1=
```

It does, however, happen for 2 or more cases and somehow gets to the very
end before segfault-ing instead of doing so by the second case
```
➜  ~ zsh -f -x -c 'zmodload zsh/param/private
function _TEST {
  private arg
  for arg in "${@}"; do
    case "${arg}" in
      test1=*)
        private test1="${arg#*=}"
      ;;

      test2=*)
        private test2="${arg#*=}"
      ;;

      test3=*)
        private test3="${arg#*=}"
      ;;
    esac
 done

 echo "test1=${test1}"
 echo "test2=${test2}"
                <....
+zsh:1> zmodload zsh/param/private
+zsh:25> _TEST 'test1=' 'test2=' 'test3='
+_TEST:1> private arg
+_TEST:2> arg=test1=
+_TEST:3> case test1= (test1=*)
+_TEST:5> private 'test1='
+_TEST:2> arg=test2=
+_TEST:3> case test2= (test1=*)
+_TEST:3> case test2= (test2=*)
+_TEST:9> private 'test2='
+_TEST:2> arg=test3=
+_TEST:3> case test3= (test1=*)
+_TEST:3> case test3= (test2=*)
+_TEST:3> case test3= (test3=*)
+_TEST:13> private 'test3='
[1]    3154406 segmentation fault (core dumped)  zsh -f -x -c
```

One easy workaround is to declare the `private` variables outside the for
loop as seen below
```
➜  ~ zsh -f -x -c 'zmodload zsh/param/private
function _TEST {
  private arg test1 test2
  for arg in "${@}"; do
    case "${arg}" in
      test1=*)
        test1="${arg#*=}"
      ;;

      test2=*)
        test2="${arg#*=}"
      ;;
    esac
 done

 echo "test1=${test1}"
 echo "test2=${test2}"
}

_TEST test1="" test2=""'
+zsh:1> zmodload zsh/param/private
+zsh:20> _TEST 'test1=' 'test2='
+_TEST:1> private arg test1 test2
+_TEST:2> arg=test1=
+_TEST:3> case test1= (test1=*)
+_TEST:5> test1=''
+_TEST:2> arg=test2=
+_TEST:3> case test2= (test1=*)
+_TEST:3> case test2= (test2=*)
+_TEST:9> test2=''
+_TEST:14> echo 'test1='
test1=
+_TEST:15> echo 'test2='
test2=
```
but I still think it's a bug because why would the single case scenario
work but not the multi-case scenario?

I also tried `local -P` instead of `private` but it yielded the same
segfault
```
➜  ~ zsh -f -x -c 'zmodload zsh/param/private
function _TEST {
  local -P arg
  for arg in "${@}"; do
    case "${arg}" in
      test1=*)
        local -P test1="${arg#*=}"
      ;;

      test2=*)
        local -P test2="${arg#*=}"
      ;;
    esac
 done

 echo "test1=${test1}"
 echo "test2=${test2}"
}

_TEST test1="" test2=""'
+zsh:1> zmodload zsh/param/private
+zsh:20> _TEST 'test1=' 'test2='
+_TEST:1> local -P arg
+_TEST:2> arg=test1=
+_TEST:3> case test1= (test1=*)
+_TEST:5> local -P test1=''
+_TEST:2> arg=test2=
+_TEST:3> case test2= (test1=*)
+_TEST:3> case test2= (test2=*)
+_TEST:9> local -P test2=''
[1]    3158070 segmentation fault (core dumped)  zsh -f -x -c
```

The `local` keyword works fine but it obviously doesn't limit the scope of
the variables as much as `private`
```
➜  ~ zsh -f -x -c 'zmodload zsh/param/private
function _TEST {
  local arg
  for arg in "${@}"; do
    case "${arg}" in
      test1=*)
        local test1="${arg#*=}"
      ;;

      test2=*)
        local test2="${arg#*=}"
      ;;
    esac
 done

 echo "test1=${test1}"
 echo "test2=${test2}"
}

_TEST test1="" test2=""'
+zsh:1> zmodload zsh/param/private
+zsh:20> _TEST 'test1=' 'test2='
+_TEST:1> local arg
+_TEST:2> arg=test1=
+_TEST:3> case test1= (test1=*)
+_TEST:5> local test1=''
+_TEST:2> arg=test2=
+_TEST:3> case test2= (test1=*)
+_TEST:3> case test2= (test2=*)
+_TEST:9> local test2=''
+_TEST:14> echo 'test1='
test1=
+_TEST:15> echo 'test2='
test2=
```

I don't know if this bug has been reported before, so please do reply to
this email if it is already being worked on (it would also be nice to know
if it was a new find).
Otherwise, I hope to see this fixed some time in the future.

Best regards,
TAN Wei Xin, Alez

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