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This program is enough to send GHC into a tizzy:

{-# LANGUAGE UnboxedSums #-}
module Bug1 where

import Type.Reflection

floopadoop :: TypeRep (# Bool | Int #)
floopadoop = typeRep

$ ghc Bug.hs
[1 of 1] Compiling Bug1             ( Bug.hs, Bug.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.4.1 for x86_64-unknown-linux):
        tyConRep
  (#|#)
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable
        pprPanic, called at compiler/deSugar/DsBinds.hs:1314:5 in ghc:DsBinds

If you use an unboxed sum data constructor, you can get a different panic:

{-# LANGUAGE PartialTypeSignatures #-}                                                             
{-# LANGUAGE TemplateHaskell #-}                                                                   
{-# LANGUAGE TypeInType #-}                                                                        
{-# LANGUAGE UnboxedSums #-}                                                                       
module Bug2 where

import Language.Haskell.TH
import Type.Reflection

type Fweemp = $(conT (unboxedSumDataName 1 2))

doopafloop :: _ => TypeRep Fweemp
doopafloop = typeRep

$ ghc Bug2.hs
[1 of 1] Compiling Bug2             ( Bug2.hs, Bug2.o )
GHC error in desugarer lookup in Bug2:
  Can't find interface-file declaration for variable $tc'(#_|#)
    Probable cause: bug in .hi-boot file, or inconsistent .hi file
    Use -ddump-if-trace to get an idea of which file caused the error
ghc: panic! (the 'impossible' happened)
  (GHC version 8.4.1 for x86_64-unknown-linux):
        initDs

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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