[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:  8.0.1
           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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 Reproduction:

 {{{#!hs
 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>
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