commas in arithmetic expansions are non-POSIX

[Rudxain]

For bugs with existing features

• My shellcheck version: 0.11.0
• I tried on https://www.shellcheck.net/ and verified that this is still a problem on the latest commit

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

#!/bin/sh
: $((x=1,y=2))

Here's what shellcheck currently says:

In tmp line 2:
: $((x=1,y=2))
      ^-- SC2034 (warning): x appears unused. Verify use (or export if used externally).
          ^-- SC2034 (warning): y appears unused. Verify use (or export if used externally).

For more information:
  https://www.shellcheck.net/wiki/SC2034 -- x appears unused. Verify use (or ...

Here's what I wanted or expected to see:

Some warning in the range SC30*, saying that the comma operator is not guaranteed by POSIX. I get an error in dash, but not bash