[GHC] #8553: ASSERT failed compiler/typecheck/TcMType.lhs line 809

[GHC]

#8553: ASSERT failed compiler/typecheck/TcMType.lhs line 809
-------------------------------------+-------------------------------------
       Reporter:  nomeata            |             Owner:
           Type:  bug                |            Status:  new
       Priority:  normal             |         Milestone:
      Component:  Compiler (Type     |           Version:  7.6.3
  checker)                           |  Operating System:  Unknown/Multiple
       Keywords:                     |   Type of failure:  Compile-time
   Architecture:  Unknown/Multiple   |  crash
     Difficulty:  Unknown            |         Test Case:
     Blocked By:                     |          Blocking:
Related Tickets:                     |
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 With current master (9c2dade94fae5f4fe19cd972ca1f1049c633e4ed) I get

 {{{
 [1 of 1] Compiling ShouldCompile    ( ../ghc/gadt6.hs, ../ghc/gadt6.o )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 7.7.20131121 for x86_64-unknown-linux):
         ASSERT failed!
     file compiler/typecheck/TcMType.lhs line 809
     [W] cobox_aBx{v} [lid] :: t_aBv{tv} [tau[0]]
                               ghc-prim:GHC.Types.~{(w) tc 31Q} t_aBu{tv}
 [tau[0]] (CIrredEvCan)
 }}}

 (And it took me a while to figure out that it was not my code that was
 introducing the bug.)

 According to `git log -L 809,809:compiler/typecheck/TcMType.lhs` (a useful
 feature) the history of the ASSERT is:

  * SPJ, “Accommodate Derived constraints in two places”, Sep 17,
 8cfbdccb7f8716c2b6b13b3c7884f52b1087d782
  * SPJ, “Fix an ASSERT”, Oct 1 2012,
 adb6bcb7b85410dd42828a4824962ce9c6e9d667
  * SPJ, “Tidy up and simplify TcMType.zonkFlats”, Sep 17, 2012,
 bd6b183e092fc7667fc55b3aa15d857f257ec73f

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8553>
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