In POSIX sh, >& is undefined when using variables representing file descriptors

[Earnestly]

For bugs

• Rule Id (if any, e.g. SC1000): SC2039
• My shellcheck version (shellcheck --version or "online"): 0.6.0
• The rule's wiki page does not already cover this (e.g. https://shellcheck.net/wiki/SC2086)
• I tried on shellcheck.net and verified that this is still a problem on the latest commit

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/sh
fd=3
exec 3>&1
printf 'test\n' >&"$fd"

Here's what shellcheck currently says:

In t.sh line 4:
printf 'test\n' >&"$fd"
                ^-----^ SC2039: In POSIX sh, >& is undefined.

For more information:
  https://www.shellcheck.net/wiki/SC2039 -- In POSIX sh, >& is undefined.

Here's what I wanted or expected to see:

Nothing.

[0xallie]

I just ran into this issue as well.