Catch typo with :offset expansion

[dimo414]

For bugs

• Rule Id (if any, e.g. SC1000):
• My shellcheck version (shellcheck --version or 'online'): 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:

#!/bin/bash
echo "${1:missing arg}"

Here's what shellcheck currently says:

No issues detected!

Here's what I wanted or expected to see:

I found a typo where I'd intended to write ${1:?missing arg} but left off the ?. This erroneously turns the expansion into an offset expansion, which (AFAICT) silently ignores non-numeric values.

It would be great if ShellCheck could flag usages of ${var:...} where ... isn't a numeral (or a variable).