[GHC] #1316: add warning for local type signatures that use the same type variable names as outer type signatures

[GHC]

#1316: add warning for local type signatures that use the same type variable names
as outer type signatures
---------------------------------+------------------------------------------
    Reporter:  Isaac Dupree      |        Owner:  simonpj     
        Type:  feature request   |       Status:  new         
    Priority:  normal            |    Milestone:  _|_         
   Component:  Compiler          |      Version:  6.6.1       
    Keywords:                    |   Difficulty:  Unknown     
          Os:  Unknown/Multiple  |     Testcase:              
Architecture:  Unknown/Multiple  |      Failure:  None/Unknown
---------------------------------+------------------------------------------
Changes (by igloo):

  * failure:  => None/Unknown


Comment:

 See also #3691.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1316#comment:8>
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