[GHC] #11427: TypeSynonyms not deduced

[GHC]

#11427: TypeSynonyms not deduced
-------------------------------------+-------------------------------------
           Reporter:  phadej         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{
 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

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11427>
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