[GHC] #5684: ghc: panic! (the 'impossible' happened) (ghc 7.2.2)

[GHC]

#5684: ghc: panic! (the 'impossible' happened) (ghc 7.2.2)
-----------------------------------------------+----------------------------
    Reporter:  selinger                        |        Owner:                    
        Type:  bug                             |       Status:  closed            
    Priority:  normal                          |    Component:  Compiler          
     Version:  7.2.1                           |   Resolution:  fixed             
    Keywords:                                  |           Os:  Unknown/Multiple  
Architecture:  Unknown/Multiple                |      Failure:  Compile-time crash
    Testcase:  typecheck/should_fail/T5684.hs  |    Blockedby:                    
    Blocking:                                  |      Related:                    
-----------------------------------------------+----------------------------
Changes (by dimitris):

  * status:  new => closed
  * testcase:  => typecheck/should_fail/T5684.hs
  * resolution:  => fixed


Comment:

 Thanks for the feedback. There were two things wrong in 7.2.1:
   - An assertion was just false because of a broken invariant
   - But, the assertion was hidden under a thunk and got triggered not at
 the right place, potentially under random poking from who-knows-who.

 Now, in HEAD by construction this invariant is true (so the real bug you
 observed no longer manifests there). But the assertion is still there so I
 did push a patch that makes the assertion trigger aggressively.

 The test is in typecheck/should_fail/T5684.hs
 and the comments in that test describe the problem in more technical
 terms.

 The relevant commit is:

 commit bd13338da1649600b6a63ae7cbf60a35575940dc
 Author: Dimitrios Vytiniotis <dimitris at microsoft.com>
 Date:   Thu Dec 15 14:18:31 2011 +0000

     Exposing a useful assertion which was hidden inside a lazyness cloud.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5684#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler