[bug] Math evaluation done twice in subscript in specific conditions

[Sebastian Gniazdowski <sgniazdowski@xxxxxxxxx>]

Hello
This is interesting:

idx=0
a=()
: ${a[++idx]::=$idx}
print -rl $a
-> 1
: ${a[++idx]::=$idx}
print -rl $a
-> 1
-> 3
idx=1
: ${a[++idx]::=$idx}
print -rl $a
-> 1
-> 2
-> 3

So apparently the assignment to idx does change something for the
following ::= "call" and makes it work correctly. The second and
following ::= calls cause $idx to increment twice.

-- 
Sebastian Gniazdowski
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