If you know of any typechecker plugin authors I've missed, please add them to the thread. In the comments of ticket https://gitlab.haskell.org/ghc/ghc/issues/15147 several of us agreed that the behavior of the typechecker plugin interface should change: GHC should no longer unflatten the fmvs in the Wanteds before passing them to the plugin. This is presumably a breaking change to plugins. We might be able to get by with some sort of flag indicating if the plugin expects the Wanteds flattened or not, but ideally GHC would just always pass them flattened. I'm unaware of any established policy about interface changes at this level, whether we've somehow committed to backwards-compatibility here or not. Anyone know? Plugin authors: would you look over the ticket comments and share your thoughts here? We're looking to build some sort of consensus about how to proceed without shocking the API users. Thank you for your time. -Nick