[GHC] #4361: Typechecker regression

[GHC]

#4361: Typechecker regression
---------------------------------------------+------------------------------
  Reporter:  igloo                           |          Owner:  simonpj         
      Type:  bug                             |         Status:  closed          
  Priority:  high                            |      Milestone:  7.0.1           
 Component:  Compiler (Type checker)         |        Version:  7.1             
Resolution:  fixed                           |       Keywords:                  
        Os:  Unknown/Multiple                |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown                    |     Difficulty:  Unknown         
  Testcase:  typecheck/should_compile/T4361  |      Blockedby:                  
  Blocking:                                  |        Related:                  
---------------------------------------------+------------------------------
Changes (by simonpj):

  * difficulty:  => Unknown


Comment:

 For completeness, `Note [Avoid unecessary constraint simplification]` is
 this:
 {{{
 Note [Avoid unecessary constraint simplification]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 When inferring the type of a let-binding, with simplifyInfer,
 try to avoid unnecessarily simplifying class constraints.
 Doing so aids sharing, but it also helps with delicate
 situations like

    instance C t => C [t] where ..

    f :: C [t] => ....
    f x = let g y = ...(constraint C [t])...
          in ...
 When inferring a type for 'g', we don't want to apply the
 instance decl, because then we can't satisfy (C t).  So we
 just notice that g isn't quantified over 't' and partition
 the contraints before simplifying.

 This only half-works, but then let-generalisation only half-works.
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4361#comment:2>
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