Currently, the defaulting plugin API matches exactly the signature of the built-in defaulting mechanism, where each type variable occuring in the `Wanteds` can be proposed to be defaulted to a priority list of types, one by one, with some extra constraints that need to hold after defaulting. Since each new constraint is associated with a single defaultable type variable, this makes it impossible to propose defaults for multi-parameter typeclasses `C a b`, or even constraints with multiple as-yet-unsolved type variables`C (T a b)`. However, it would not take much to extend the plugin API to support this use case. I have a prototype implementation at https://gitlab.haskell.org/cactus/ghc/-/commit/b73d407b20c2ba8c0792855b41d1b5dd5dc1aee5 that changes the API to this: ``` data DefaultingProposal = DefaultingProposal { deProposalTyVars :: [TcTyVar] -- ^ The type variable to default. , deProposalCandidates :: [[Type]] -- ^ Candidate types to default the type variable to. , deProposalCts :: [Ct] -- ^ The constraints against which defaults are checked. } ``` The types could be a bit tighter, because basically we want a rectangle of proposal candidates (`all (equalLength deProposalTyVars) deProposalCandidates`), but I don't know how that's usually done in the GHC codebase so for the prototype this shall do.