Panic with (bogus?) deriving in hs-boot: newTyConEtadArity

$ cat A.hs
module Main where

import {-# SOURCE #-} B

main = return ()
$ cat B.hs
module B where

data Foo a = Foo a
  deriving (Functor)
$ cat B.hs-boot
module B where

data Foo a
  deriving (Functor)
$ ghc --make A.hs
[1 of 3] Compiling B[boot]          ( B.hs-boot, B.o-boot )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.2.1 for i386-unknown-linux):
	newTyConEtadArity
  Foo
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable
        pprPanic, called at compiler/types/TyCon.hs:2149:27 in ghc:TyCon

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

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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