superclasses aren't considered because context is no smaller than the instance head

class (AllF f xs, SListI xs) => All (f :: k -> Constraint) (xs :: [k])

instance
#if __GLASGOW_HASKELL__ >= 710
  {-# OVERLAPPING #-}
#endif
  All SListI xss => SingI (xss :: [[k]]) where
  sing = sList

fails with

    • Could not deduce (SListI xss)
        arising from the superclasses of an instance declaration
      from the context: All SListI xss
        bound by the instance declaration
        at src/Generics/SOP/Constraint.hs:141:3-40
    • In the instance declaration for ‘SingI xss’

See https://travis-ci.org/well-typed/generics-sop/jobs/102388817 and https://github.com/phadej/generics-sop/tree/ghc-8.0.1

Trac metadata

Trac field	Value
Version	8.0.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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