Feature request: Empty associative-array keys

[Koichi Murase]

This is a topic I originally mentioned in another discussion on the
new treatment of assoc_expand_once (
https://lists.gnu.org/archive/html/bug-bash/2021-03/msg00059.html ).
As in the mail subject, I suggest supporting empty keys for
associative arrays.  Does anyone have any opinion on this suggestion?

Description:

Currently, associative arrays do not accept empty keys.  We can work
it around e.g. by always prefixing `x' before the keys, such as
`assoc[x$key]=$value'.  But I still think we may support empty keys
for associative arrays because I don't see strong reasons to disallow
the empty keys of associative arrays.  Here is the quote of the
original discussion:

2021年3月16日(火) 10:41 Koichi Murase <myoga.murase@gmail.com>:
> Maybe this is unrelated to the extra expansions of array subscripts,
> but is there any reason that Bash disallows empty associative keys?
> This is also a point that confuses users related to the keys of
> associative arrays. Associative arrays in other languages usually
> accept an empty key. I usually use `a[x$key]' for the workaround, but
> I personally feel we should support empty associative keys in Bash. I
> also remember that at least two other people were confused by the
> current behavior not accepting the empty key.

Repeat-By:

Here I summarize the current behavior for the empty keys.  It seems
Bash already parses empty keys for assignments, but they seem to be
afterward rejected:
  $ declare -A a=()
  $ a[]=1
  bash-dev: a[]: bad array subscript
  $ a['']=1
  bash-dev: a['']: bad array subscript

`a[]' in parameter expansions are not parsed.  The variant `a['']'
pass through the parsing phase but is later rejected:
  $ echo ${a[]}
  bash-dev: ${a[]}: bad substitution
  $ echo ${a['']}
  bash-dev: a: bad array subscript

The variable name `a[]' is rejected in `printf -v', indirect
expansions, and namerefs:
  $ printf -v 'a[]' 1
  bash-dev: printf: `a[]': not a valid identifier
  $ printf -v 'a[""]' 1
  bash-dev: a[""]: bad array subscript
  $ iref='a[]'; echo ${!iref}
  bash-dev: a[]: invalid variable name
  $ iref='a[""]'; echo ${!iref}
  bash-dev: a: bad array subscript
  $ declare -n nref='a[]'; echo $nref
  bash-dev: declare: `a[]': invalid variable name for name reference
  $ declare -n nref='a[""]'; echo $nref
  bash-dev: a: bad array subscript

No error message for `unset 'a[]''?
  $ unset 'a[]'
  $ unset 'a[""]'
  bash-dev: unset: [""]: bad array subscript

--
Koichi