segfault evaluating [[ $@ ]] when IFS is \t or \n and $1 == $IFS

[Grisha Levit]

Given:

set -- $'\t'; IFS=$1 A=$1   # or set -- $'\n'

These all segfault (in different places with -DDEBUG)

: ${x-$@}
[[ $@ ]]
: ${x-${A[@]}}
[[ ${A[@]} ]]