Failure compiling ghc-mtl with ghc-7.8.{2,3}

[i hamsa]

I was trying to upgrade to ghc-7.8 the other day, and got this
compilation failure when building ghc-mtl-1.2.1.0 (see the end of the
message).

I'm using the haskell overlay on Gentoo Linux straight out of the box,
no local cabal installations of anything.

Now I was told that other people can compile ghc-mtl with 7.8 just
fine, so there must be something broken in my specific configuration.
What would be an effective way to approach the situation?

In the sources I see that an instance of MonadIO GHC.Ghc does exist. I
don't understand these errors. Are there multiple different MonadIO
classes in different modules?

Thank you and happy hacking.

Now the errors:

Control/Monad/Ghc.hs:42:15:
    No instance for (GHC.MonadIO Ghc)
      arising from the 'deriving' clause of a data type declaration
    Possible fix:
      use a standalone 'deriving instance' declaration,
        so you can specify the instance context yourself
    When deriving the instance for (GHC.ExceptionMonad Ghc)

Control/Monad/Ghc.hs:46:15:
    No instance for (MonadIO GHC.Ghc)
      arising from the 'deriving' clause of a data type declaration
    Possible fix:
      use a standalone 'deriving instance' declaration,
        so you can specify the instance context yourself
    When deriving the instance for (MonadIO Ghc)

Control/Monad/Ghc.hs:49:15:
    No instance for (GHC.MonadIO Ghc)
      arising from the 'deriving' clause of a data type declaration
    Possible fix:
      use a standalone 'deriving instance' declaration,
        so you can specify the instance context yourself
    When deriving the instance for (GHC.GhcMonad Ghc)

-- 
this is the real i