Standalone deriving of Functor does not work for HsDoc

I tried to use the new DerivingFunctor extension to derive Functor for the HsDoc type from the ghc package:

{-# StandaloneDeriving, DerivingFunctor #-}

import HsDoc

deriving instance Functor HsDoc

But I got this error message:

david@david-laptop:~/dev/test$ /home/david/ghc-head/bin/ghc --make Test.hs -package ghc
[1 of 1] Compiling Test             ( Test.hs, Test.o )

Test.hs:9:0:
    Couldn't match expected type `b' against inferred type `a'
      `b' is a rigid type variable bound by
          the type signature for `fmap' at <no location info>
      `a' is a rigid type variable bound by
          the type signature for `fmap' at <no location info>
      Expected type: HsDoc b
      Inferred type: HsDoc a
    In the first argument of `DocAppend', namely `a1'
    In the expression: DocAppend a1 a2

Trac metadata

Trac field	Value
Version	6.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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