[GHC] #15778: GHC HEAD-only panic (zonkTcTyVarToTyVar)

[GHC]

#15778: GHC HEAD-only panic (zonkTcTyVarToTyVar)
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.7
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I know what is happening here.  In `tcHsPatSigType` we have
 {{{
     mk_tv_pair tv = do { tv' <- zonkTcTyVarToTyVar tv
                        ; return (tyVarName tv, tv') }
 }}}
 But now the tyvar might have unified with a ''type'' not a type
 ''variable''.  And in these cases it does.  I think that the right thing
 is simply to remove the zonk.  I'll act on this.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15778#comment:5>
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