Possible bug with ${(A)foo=} and ${(AA)foo=}

["Andrej Borsenkow" <borsenkow.msk@sni.de>]

While playing with something else I discovered, that it is impossible to create
empty array with this form and it is impossible to create hash in this way
without error message:

bor@itsrm2:~%> unset foo
bor@itsrm2:~%> : ${(A)foo=}
bor@itsrm2:~%> print ${#foo}
1

bor@itsrm2:~%> unset foo
bor@itsrm2:~%> : ${(A)foo=""}
bor@itsrm2:~%> print ${#foo}
1

bor@itsrm2:~%> unset foo
bor@itsrm2:~%> : ${(AA)foo=""}
zsh: bad set of key/value pairs for associative array

bor@itsrm2:~%> print ${#foo}
0

bor@itsrm2:~%> : ${(AA)foo=}
zsh: bad set of key/value pairs for associative array

bor@itsrm2:~%> print ${#foo}
0

I think, that if ${foo=} is valid at all, then we must distinguish between no
value and empty value. It is the same as in command line (and, after all, we
compute it using the same rules?) - empty value is lost but explict NULL like ""
is retained. In other words,

${(A)foo=} - creates empty array
${(A)foo=""} - creates array with one element of length 0
${(AA)foo=} - creates empty hash
${(AA)foo=""} - is invalid assignment as expected

[Background - ${(AA)foo=} seems currently to be the only way to create global
hash from within Zsh function]

/andrej