[GHC] #16123: QuantifiedConstraints fails to deduce trivial constraint

[GHC]

#16123: QuantifiedConstraints fails to deduce trivial constraint
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        Reporter:  eschnett          |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:  duplicate         |             Keywords:
                                     |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14680            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by eschnett):

 For the record: I have a working work-around using {{{Data.Constraint}}}.
 The {{{functor}}} class then reads
 {{{
 -- | A functor
 class (Category (Dom f), Category (Cod f)) => Functor f where
   -- | Prove that this functor maps from its domain to its codomain
   proveFunctor :: Ok (Dom f) a => Ok (Dom f) a :- Ok (Cod f) (f a)
   type Dom f :: CatKind
   type Cod f :: CatKind
   fmap :: (Ok (Dom f) a, Ok (Dom f) b) => Dom f a b -> Cod f (f a) (f b)
 }}}

 and the previously-failing instance declaration becomes
 {{{
 instance (Functor f, Dom f ~ NFun, Cod f ~ NFun) => Functor (NIdF f) where
   type Dom (NIdF f) = Dom f
   type Cod (NIdF f) = Cod f
   proveFunctor :: forall a. Ok (Dom (NIdF f)) a
                => Ok (Dom (NIdF f)) a :- Ok (Cod (NIdF f)) ((NIdF f) a)
   proveFunctor = Sub Dict
                  \\ (proveFunctor @f :: Ok (Dom f) a :- Ok (Cod f) (f a))
   fmap f = NFun \(NIdF xs) -> NIdF (neval (fmap f) xs)
 }}}

 The disadvantage is that {{{proveFunctor}}} is now an explicit function,
 and using {{{fmap}}} now might require calling {{{proveFunctor}}}
 explicitly to invoke it as proof.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16123#comment:2>
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