inferred contexts from `deriving`

Hi devs, I've just hit on (what I view to be) a design wart in what is allowed in an instance context inferred through the use of non-standalone `deriving`. I've posed a problem and potential solution in #8984 (https://ghc.haskell.org/trac/ghc/ticket/8984#comment:6). See Note [Exotic derived instance contexts] (https://github.com/ghc/ghc/blob/master/compiler/typecheck/TcDeriv.hs#L1915) for some background info. I would love input on this issue, as it all boils down to a simple design choice. The implementation is dead easy no matter what. Please post any further comments to the ticket, but I wanted to bleat here because it seems easy to ignore emails from #8984, whose title is about tweaking an error message. Thanks, Richard