[GHC] #7541: Unavoidable duplicate constraint warning

[GHC]

#7541: Unavoidable duplicate constraint warning
----------------------------------------------+-----------------------------
Reporter:  blamario                           |          Owner:                  
    Type:  bug                                |         Status:  new             
Priority:  normal                             |      Component:  Compiler        
 Version:  7.6.1                              |       Keywords:                  
      Os:  Unknown/Multiple                   |   Architecture:  Unknown/Multiple
 Failure:  Incorrect warning at compile-time  |      Blockedby:                  
Blocking:                                     |        Related:                  
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 The "duplicate constraint" warning can be triggered in a context where
 it's impossible to remove it, because the constraints come with a type
 synonym.

 The following standalone little module reproduces the problem:

 {{{
 {-# LANGUAGE FlexibleContexts, Rank2Types #-}

 module Test where

 type Constrained x y r = (Eq x, Eq y) => x -> y -> r

 f :: Constrained String String ()
 f = undefined
 }}}

 The warning is

 {{{
 Test.hs:7:6: Warning:
     Duplicate constraint(s): Eq String
     In the type signature for `f': f :: Constrained String String ()
 }}}

 The warning is present in 7.4.1 and 7.6.1, and presumably in the versions
 between as well.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7541>
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