[GHC] #11515: PartialTypeSignatures suggests a redundant constraint with constraint families

[GHC]

#11515: PartialTypeSignatures suggests a redundant constraint with constraint
families
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           Reporter:  goldfire       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           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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 When I say

 {{{
 {-# LANGUAGE ConstraintKinds, TypeFamilies #-}

 module Bug where

 type family ShowSyn a where ShowSyn a = Show a

 foo :: (ShowSyn a, _) => a -> String
 foo x = show x
 }}}

 I get

 {{{
 Bug.hs:7:20: error:
     Found constraint wildcard ‘_’ standing for ‘Show a’
     To use the inferred type, enable PartialTypeSignatures
     In the type signature:
       foo :: (ShowSyn a, _) => a -> String
 }}}

 But if I fill in my wildcard with `Show a`, I'll rightly get a redundant
 constraint. Just a vanilla type synonym works there, but the type family
 causes trouble.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11515>
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