[GHC] #16127: Panic: piResultTys1 in compiler/types/Type.hs:1022:5

[GHC]

#16127: Panic: piResultTys1 in compiler/types/Type.hs:1022:5
-------------------------------------+-------------------------------------
           Reporter:  _deepfire      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
  (Type checker)                     |
           Keywords:                 |  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:
-------------------------------------+-------------------------------------
 I apologize in advance for a non-minimised repro.

 The panic (with a small `-ddump-tc-trace` excerpt, but full log also
 attached):

 {{{
 tcPolyExprNC
   Check{Vocab
           *
           (k_a4TNy[tau:2] -> Constraint)
           i_a4TNv[tau:2]
           (Present * k_a4TNy[tau:2] i_a4TNv[tau:2])}
 tcSkolemise
 tcInferId
   voc_a4TB2 :: Vocab i_a4TMf[sk:1] (Present * * i_a4TMf[sk:1])
 tcCheckId
   voc_a4TB2
   Vocab i_a4TMf[sk:1] (Present * * i_a4TMf[sk:1])
   Check{Vocab
           *
           (k_a4TNy[tau:2] -> Constraint)
           i_a4TNv[tau:2]
           (Present * k_a4TNy[tau:2] i_a4TNv[tau:2])}
 tcWrapResult
   Actual:   Vocab i_a4TMf[sk:1] (Present * * i_a4TMf[sk:1])
   Expected: Check{Vocab
                     *
                     (k_a4TNy[tau:2] -> Constraint)
                     i_a4TNv[tau:2]
                     (Present * k_a4TNy[tau:2] i_a4TNv[tau:2])}
 tc_sub_type_ds
   ty_actual   = Vocab i_a4TMf[sk:1] (Present * * i_a4TMf[sk:1])
   ty_expected = Vocab
                   *
                   (k_a4TNy[tau:2] -> Constraint)
                   i_a4TNv[tau:2]
                   (Present * k_a4TNy[tau:2] i_a4TNv[tau:2])
 deeply_instantiate final subst
   origin: arising from a use of ‘voc_a4TB2’
   type: Vocab i_a4TMf[sk:1] (Present * * i_a4TMf[sk:1])
   new type: Vocab i_a4TMf[sk:1] (Present * * i_a4TMf[sk:1])
   subst: [TCvSubst
             In scope: InScope {i_a4TMf}
             Type env: []
             Co env: []]
 u_tys
   tclvl 2
   Vocab i_a4TMf[sk:1] (Present * * i_a4TMf[sk:1])
   ~
   Vocab
     *
     (k_a4TNy[tau:2] -> Constraint)
     i_a4TNv[tau:2]
     (Present * k_a4TNy[tau:2] i_a4TNv[tau:2])
   arising from a type equality Vocab
                                  i_a4TMf[sk:1] (Present * * i_a4TMf[sk:1])
                                ~
                                Vocab
                                  *
                                  (k_a4TNy[tau:2] -> Constraint)
                                  i_a4TNv[tau:2]
                                  (Present * k_a4TNy[tau:2] i_a4TNv[tau:2])

 <no location info>: error:
     ghc: panic! (the 'impossible' happened)
   (GHC version 8.6.1 for x86_64-unknown-linux):
         piResultTys1
   *
   [i_a4TNv[tau:2], Present * k_a4TNy[tau:2] i_a4TNv[tau:2]]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/types/Type.hs:1022:5 in ghc:Type

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 Repro (requires Nix & some download time, provides hard repro guarantees
 in return):

 {{{
 $ git clone https://github.com/deepfire/holotype
 $ cd holotype
 $ git reset --hard 7e9f5099eea2080f57205267291eeec50ae4b68a
 $ nix-shell
 [nix-shell:~/holotype]$ make
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16127>
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