[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:                  
  Testcase:  typecheck/should_compile/T4361  |      Blockedby:                  
Difficulty:                                  |             Os:  Unknown/Multiple
  Blocking:                                  |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown                    |  
---------------------------------------------+------------------------------
Changes (by simonpj):

  * status:  new => closed
  * testcase:  => typecheck/should_compile/T4361
  * resolution:  => fixed


Comment:

 Aha. It turns out that Sergei is pushing the boundaries here.  It's not at
 all clear that this program should be accepted.  Still I've beefed up
 `simplifyInfer` so that it does the job.  I had to take two bites at this
 particular cherry:
 {{{
 Fri Oct  8 04:12:27 PDT 2010  simonpj at microsoft.com
   * Fix Trac #4361: be more discerning when inferring types

   Note [Avoid unecessary constraint simplification] in TcSimplify

     M ./compiler/typecheck/TcSimplify.lhs -3 +41
 }}}
 and
 {{{
 Fri Oct  8 06:35:42 PDT 2010  simonpj at microsoft.com
   * Do less simplification when doing let-generalisation

   This fixes Trac #4361.  In a rather delicate way, but
   no more delicate than before.  A more remoseless typechecker
   would reject #4361 altogether.

   See Note [Avoid unecessary constraint simplification]

     M ./compiler/typecheck/TcSimplify.lhs -22 +32
 }}}
 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4361#comment:1>
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