[GHC] #11474: incorrect redundant-constraints warning

[GHC]

#11474: incorrect redundant-constraints warning
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           Reporter:  hvr            |             Owner:
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  8.0.1-rc1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The following code when compiled with GHC 8

 {{{#!hs
 {-# LANGUAGE Haskell2010, FunctionalDependencies, KindSignatures,
              ScopedTypeVariables, TypeOperators, UndecidableInstances #-}

 import GHC.Generics

 data Options
 data Value

 newtype Tagged s b = Tagged {unTagged :: b}

 class GToJSON f where
     gToJSON :: Options -> f a -> Value

 class SumToJSON f allNullary where
     sumToJSON :: Options -> f a -> Tagged allNullary Value

 class AllNullary (f :: * -> *) allNullary | f -> allNullary

 instance (
     AllNullary (a :+: b) allNullary, -- <- removing this line causes a
 compile error
     SumToJSON  (a :+: b) allNullary )
     => GToJSON (a :+: b) where

     gToJSON opts = (unTagged :: Tagged allNullary Value -> Value) .
 sumToJSON opts
 }}}

 emits a warning

 {{{
 bug.hs:19:10: warning:
     • Redundant constraint: AllNullary (a :+: b) allNullary
     • In the instance declaration for ‘GToJSON (a :+: b)’
 }}}

 when commenting out the `AllNullary` constraint, this however results the
 compile error

 {{{
 bug.hs:19:10: error:
     • Could not deduce (SumToJSON (a :+: b) allNullary0)
       from the context: SumToJSON (a :+: b) allNullary
         bound by an instance declaration:
                    SumToJSON (a :+: b) allNullary => GToJSON (a :+: b)
         at redconstr.hs:(19,10)-(22,24)
       The type variable ‘allNullary0’ is ambiguous
     • In the ambiguity check for an instance declaration
       To defer the ambiguity check to use sites, enable
 AllowAmbiguousTypes
       In the instance declaration for ‘GToJSON (a :+: b)’
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11474>
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