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{-# LANGUAGE Strict #-}
main = print $ let x = undefined in True

Using HEAD or GHC 8.0:

$ ghc-8.0.0.20160109 Test.hs
[1 of 1] Compiling Main             ( Test.hs, Test.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.0.20160109 for x86_64-unknown-linux):
	StgCmmEnv: variable not found
  $dIP_aDK
  local binds for:

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

Using a devel2 build:

$ ./inplace/bin/ghc-stage2 Test.hs
[1 of 1] Compiling Main             ( Test.hs, Test.o )
WARNING: file compiler/simplCore/OccurAnal.hs, line 66
  Glomming in Main: [aDO :->]
WARNING: file compiler/coreSyn/CoreSubst.hs, line 278
  CoreSubst.lookupIdSubst simpleOptExpr $dIP_aDO
  InScope [aDM :-> a_aDM]
WARNING: file compiler/simplCore/OccurAnal.hs, line 66
  Glomming in Main: [aDO :-> Once]
WARNING: file compiler/simplCore/SimplEnv.hs, line 530 $dIP_aDO
WARNING: file compiler/simplCore/OccurAnal.hs, line 66
  Glomming in Main: [aDO :-> Once]
WARNING: file compiler/simplCore/SimplEnv.hs, line 530 $dIP_aDO
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.1.20160103 for x86_64-unknown-linux):
	ASSERT failed! file compiler/stgSyn/CoreToStg.hs line 1025 $dIP_aDO

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