If I write a bit array pattern like this one:

case todo {
  <<-2:unsigned>> -> todo
  _ -> todo
}

The compiler should warn that that branch is unreachable since it's impossible that an unsigned number matches with the value -2. This happens in all cases where we're matching on a number that can't be represented with the given size and signedness.

This not being handled properly also leads to some code generation bugs on the javascript target.