Panic on ":t datatypeName"

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