Add a special case warning for parens in for f in (a b)

[koalaman]

For bugs

• Rule Id (if any, e.g. SC1000):
• My shellcheck version (shellcheck --version or "online"):
• I tried on shellcheck.net and verified that this is still a problem on the latest commit
• It's not reproducible on shellcheck.net, but I think that's because it's an OS, configuration or encoding issue

For new checks and feature suggestions

• shellcheck.net (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Here's a snippet or screenshot that shows the problem:

It's somewhat common to try to put parens around for arguments:

for f in (a b c)
do echo "$f"; done

Examples:

for dir in ( cat foo.txt )
for file in ( file* )
for num in ( 0..100 )

Here's what shellcheck currently says:

Currently, this is caught with a generic "shell word starts with (" check:

for f in (a b c)
^-- SC1073: Couldn't parse this for loop. Fix to allow more checks.
         ^-- SC1036: '(' is invalid here. Did you forget to escape it?

But obviously it's not supposed to be escaped.

Here's what I wanted or expected to see:

Since the intent is known, a specific warning could be more helpful:

for f in (a b c)
^-- SC1073: Couldn't parse this for loop. Fix to allow more checks.
         ^-- SC1XXX: Remove invalid (..) around `for` arguments, or use `while read` to process command output.