Strange type representation bug with phantom type synonyms in interactive mode

Sorry for the confusing summary

The following module defines a phatom type synonym and a simple function

module Phantom where

type PhantomSyn a  = Int

f = (\_ -> 2) :: PhantomSyn a -> Int

When running in interactive mode (the bug doesn't seem to be reproducible otherwise) the typechecker gives a strange error

./ghc-inplace  --interactive /tmp/Phantom.hs
GHCi, version 6.9.20071025: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
[1 of 1] Compiling Phantom          ( /tmp/Phantom.hs, interpreted )
Ok, modules loaded: Phantom.
*Phantom> f "incorrectParam"

<interactive>:1:2:
    Couldn't match expected type `PhantomSyn GHC.Prim.Any'
           against inferred type `[Char]'
      Expected type: PhantomSyn GHC.Prim.Any
      Inferred type: [Char]
    In the first argument of `f', namely `"incorrectParam"'
    In the expression: f "incorrectParam"

The typechecker should output

Expected type: PhantomSyn a

If I skip the interactive mode (including the erroneous declaration in the Phantom.hs itself) or I define f as

f :: PhantomSyn a -> Int
f = (\_ -> 2)

The incorrect error report disappears.

Trac metadata

Trac field	Value
Version	6.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
Test case
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Operating system	Unknown
Architecture	Unknown