already assigned associative array elements cannot be used in default assignment of another element of same array #966

did not know how to better describe this behaviour (I use this : "${var:=val}" construct for default value assignments in a certain context since it works with set -u if var is not defined yet):

unset x
typeset -A x
x[a]=1
y=${x[a]}
: "${x[b]:=${x[a]}}"
: "${x[c]:=$y}"
print -v x

result:

(
        [a]=1
        [b]=
        [c]=1
)

so x[b] does not pick up the value of x[a] as it should (same with legacy ksh93u+, bash behaves as intended). looks like a bug?