Note that
case <foo> | [] -> ... | (_ : xs) -> ...
also contains a wildcard-pattern. So emitting a warning for every use of wildcard patterns would likely lead to a lot of pain.
Yes, good point, that would be too restrictive. When I said wildcard-pattern I was thinking specifically of a top-level wildcard, so your example would be accepted, but e.g. case ... of [] -> ... _ -> ... would be rejected.
You'd instead want to warn about "default branch", e.g.
case <foo> | [] -> ... | (1 : xs) -> ... | (_ : xs) -> ...
here the wildcard pattern does correspond to a "default branch" and might hence deserve a warning.
This sounds promising, but how would you define “default branch”? Seems like it could be an involved definition, which could make the warning unpredictable for users.