[GHC] #12549: Panic on ":t datatypeName"

[GHC]

#12549: Panic on ":t datatypeName"
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        Reporter:  johnleo       |                Owner:
            Type:  bug           |               Status:  new
        Priority:  normal        |            Milestone:
       Component:  Compiler      |              Version:  8.1
      Resolution:                |             Keywords:
Operating System:  MacOS X       |         Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:                |  Differential Rev(s):
       Wiki Page:                |
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Description changed by johnleo:

@@ -3,1 +3,1 @@
- {{{#!hs
+ {{{

New description:

 Reproduction:

 {{{
 GHCi, version 8.1.20160826: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /Users/leo/.ghci
 Prelude> :m + GHC.Generics
 :m + GHC.Generics
 Prelude GHC.Generics> :t datatypeName
 :t datatypeName
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.1.20160826 for x86_64-apple-darwin):
         ASSERT failed!
   CallStack (from HasCallStack):
   assertPprPanic, called at compiler/types/TyCoRep.hs:2094:44 in
 ghc:TyCoRep
   checkValidSubst, called at compiler/types/TyCoRep.hs:2122:17 in
 ghc:TyCoRep
   substTy, called at compiler/typecheck/TcMType.hs:793:24 in ghc:TcMType
   in_scope InScope {k1_a1X8}
   tenv [a1X0 :-> k1_a1X8[tau:3]]
   cenv []
   tys [k_a1X4[tau:3] → (k1_a1X0 → *) → k1_a1X0 → ★]
   cos []
   needInScope [a1X4 :-> k_a1X4[tau:3]]

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

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12549#comment:4>
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