Named reference (and other) pending work

*Pending patches about named references *(from most to least important)

- Remove support for named references to subscripted variables
<https://github.com/zsh-users/zsh/compare/master...paltherr:zsh:kill-subscripted-refs>
 - workers/54363 <http://zsh.org/workers/54363>
- Fix nameref issues in builtin "unset"
<https://github.com/zsh-users/zsh/compare/master...paltherr:zsh:fix-unset>
 - workers/54364 <http://zsh.org/workers/54364>
  - Implement references to positional parameters
<https://github.com/paltherr/zsh/compare/paltherr:fix-unset...paltherr:zsh:fix-argn-refs>
 - workers/54365 <http://zsh.org/workers/54365>
- Detect invalid variable names in reference initializers
<https://github.com/zsh-users/zsh/compare/master...paltherr:zsh:detect-invalid-variable-names>
 - workers/54278 <http://zsh.org/workers/54278>
- Remove unnecessary tag based reference loop detection
<https://github.com/zsh-users/zsh/compare/master...paltherr:zsh:kill-tagged-loop-detection>
 - workers/54277 <http://zsh.org/workers/54277>

*Other pending patches* (from most to least important)

- Run ZERR trap also when there was a Zsh error
<https://github.com/zsh-users/zsh/compare/master...paltherr:zsh:run-zerr-trap-on-zsh-error>
 - workers/54374 <http://zsh.org/workers/54374>
- Remove partial duplication of strsetfn and arrsetfn
<https://github.com/zsh-users/zsh/compare/master...paltherr:zsh:kill-setfn-duplication>
 - workers/54340 <http://zsh.org/workers/54340>
  - Refine string and array slice assignments
<https://github.com/paltherr/zsh/compare/fix-reversed-slices-bis...paltherr:zsh:refine-slice-assignments>
 - workers/54348 <http://zsh.org/workers/54348>
- Only ever turn global parameters into named directories
<https://github.com/paltherr/zsh/compare/kill-setfn-duplication...kill-local-nameddirs>
 - workers/54328 <http://zsh.org/workers/54328>

*Review of dereferencing lookups*

I have started reviewing all parameter lookups that also dereference named
references, i.e., more or less all calls to realparamtab->getnode().
Several of these lookups have issues. In many cases, they should simply not
perform any dereferencing (i.e., call getnode2() instead of getnode) but a
few are more problematic. I have already reviewed most lookup and will
start sending out patches and/or start discussions to figure out what to do.

*Review of typeset implementation*

A few months ago, I started an exhaustive review of the implementation of
typeset and found many issues, which lead to many side tracks. Some of them
have been resolved and a few others are among the patches listed above.
However, I know for sure that there are still issues with typeset. In
particular with -p and -m when applied to named references. I suspect that
there are also still issues with some calls to typeset p=v where p is an
existing named reference.

*Turn placeholder dereferencing into an error*

In ksh, any attempt to dereference a placeholder reference triggers an
error. Zsh should do the same. Currently it sometimes exhibits rather
dubious behaviors in place of triggering an error.

*Discuss the semantics of references to not-yet-defined variables*

With the current implementation, references to not-yet-defined variables
lead sometimes to behaviors that are nonsensical in my opinion. I think
that a different approach could lead to references that are not only less
problematic but also generally more useful.

*Other issues*

I'm aware of a number of other issues but as long as named references are
released as an experimental feature, I think that they can be addressed
after the release.

Philippe