$${a,b} brace expansion flaw.

[Kaz Kylheku]

Someone just posted this finding on Stackoverflow:

https://stackoverflow.com/questions/77850130/bash-parse-of-x-y

When brace expansion is adjacent to $$, it doesn't expand.

Try

   echo $${a,b}
   echo $$-{a,b}

Documented somewhere? Or bug?