A Shift Outside a (( Line Still Strips the Commands After It #2062

Problem

_heredoc_openers in gh-write-guard.py tells an arithmetic shift from a heredoc only by a (( token on the same line. A shift in any other arithmetic context has no such token, so its << is read as a heredoc opener and every line after it is stripped as body. A wait loop on those lines is then never judged.

Reproduce (constructed)

Each of these is allowed on develop, and each loops under timeout 3 bash until the timeout kills it:

x=$((
1 << n ))
while [ ! -f /nonexistent ]; do sleep 1; done

x=$[ 1 << n ]
while [ ! -f /nonexistent ]; do sleep 1; done

${a[ i << n ]} tokenizes the same way, to a bare << followed by n.

Done looks like

The commands above are denied, and a self-test row pins each. One route is to give such a line the same two readings a (( line now gets, keyed on the $[ or ${ token or on an arithmetic context left open by an earlier line.

Found by the local strict review of the #2059 fix (handoff #2061), which keys only on ((. It is older than that fix.