[GHC] #15552: Infinite loop/panic with an existential type.

[GHC]

#15552: Infinite loop/panic with an existential type.
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        Reporter:  howtonotwin       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:  TypeInType,
                                     |  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14723            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"565ef4cc036905f9f9801c1e775236bb007b026c/ghc" 565ef4c/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="565ef4cc036905f9f9801c1e775236bb007b026c"
 Remove knot-tying bug in TcHsSyn.zonkTyVarOcc

 There was a subtle knot-tying bug in TcHsSyn.zonkTyVarOcc, revealed
 in Trac #15552.

 I fixed it by

 * Eliminating the short-circuiting optimisation in zonkTyVarOcc,
   instead adding a finite map to get sharing of zonked unification
   variables.

   See Note [Sharing when zonking to Type] in TcHsSyn

 * On the way I /added/ the short-circuiting optimisation to
   TcMType.zonkTcTyVar, which has no such problem.  This turned
   out (based on non-systematic measurements) to be a modest win.

   See Note [Sharing in zonking] in TcMType

 On the way I renamed some of the functions in TcHsSyn:

 * Ones ending in "X" (like zonkTcTypeToTypeX) take a ZonkEnv

 * Ones that do not end in "x" (like zonkTcTypeToType), don't.
   Instead they whiz up an empty ZonkEnv.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15552#comment:10>
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