I have a module A that re-exports module B, and module B contains only class
instances. Since I'm using -Wall, I get this warning about module B
exporting nothing. Is there a way to disable this particular warning, since
it is unnecessary in this case? No mailing list messages or GHC
documentation has told me what to do.

Well, the warning is right that you don't need to re-export module B: instances are implicitly exported. So you could just remove the export of "module B", unless there's a reason to export it (such as, you might add some exported functions or data types to it later)

Hmm, the disappointing result of removing module B from the export list is that now it doesn't show up in the Haddock-generated documentation for module A. Not that there was anything specific in B to document, because it's only instances. But I do want the user to know that importing A also imports B. Sigh... I suppose there's CPP if I really want it.

Sean