[GHC] #7101: Specialise broken for implicit parameters

[GHC]

#7101: Specialise broken for implicit parameters
---------------------------------+------------------------------------------
    Reporter:  pcapriotti        |       Owner:                             
        Type:  bug               |      Status:  new                        
    Priority:  normal            |   Milestone:  7.6.1                      
   Component:  Compiler          |     Version:  7.4.2                      
    Keywords:                    |          Os:  Unknown/Multiple           
Architecture:  Unknown/Multiple  |     Failure:  Incorrect result at runtime
  Difficulty:  Unknown           |    Testcase:                             
   Blockedby:                    |    Blocking:                             
     Related:                    |  
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 The `tcrun019` test case is failing with optimization on. Here is an even
 simpler test case:

 {{{
 {-# LANGUAGE ImplicitParams, RankNTypes #-}

 module Main where

 f :: (?x :: Int) => ((?x :: Int) => Int) -> Bool -> Int
 f g False = g
 f g True = let ?x = ?x + 1
            in f g False

 h :: (?x :: Int) => Int
 h = ?x

 main :: IO ()
 main = print (let ?x = 0 in f h True)
 }}}

 What happens is that `f` gets incorrectly specialized, and, as a result,
 the
 recursive call to `f` uses the old binding for the implicit parameter.

 I think the problem was introduced with the refactoring in
 5a8ac0f823c151c062a3f1903574030423bb255c, which broke the test for
 implicit
 parameter constraints in `Specialise.lhs`.

 The attached patch seems to fix it.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7101>
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