+=() can be used to set illegal indices

[Emanuele Torre]

If an indexed array has no available slots after its last index, +=()
will overflow the last index, and set into an illegal negative index,
effectively prepending values instead of appending them.

   $ x=([9223372036854775807]=hello)
   $ x+=( and hi )
   $ echo "${x[@]}"
   and hi hello
   $ x+=( {a..f} ) # it overwrites the invalid indices, since the MAX index is 
the same
   $ echo "${x[@]}"
   a b c d e f hello

   $ x=([9223372036854775805]=foo)
   $ x+=( {1..5} )
   $ echo "${x[@]}"
   3 4 5 foo 1 2

This also makes the output of declare -p invalid

   $ x=([9223372036854775807]=abc) x+=( xyz )
   $ declare -p x
   declare -a x=([-9223372036854775808]="xyz" [9223372036854775807]="abc")
   $ declare -a x=([-9223372036854775808]="xyz" [9223372036854775807]="abc")
   bash: [-9223372036854775808]=xyz: bad array subscript

I think if  INTMAX_MAX - last_index < elements_to_append  , bash should
just error (error similar to setting to a readonly variable).

Another approach could be at least overflowing to 0 instead of a
negative number, so, at least, the output of declare -p is still valid,
but I would prefer that it just errors before appending anything.

o/
  emanuele6