Poor warning for conflicting functional dependencies

E:\Neil\thesis>ghci obj\haskell2\Proof_default.hs
GHCi, version 6.8.2: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
[1 of 1] Compiling Proof_default    ( obj/haskell2/Proof_default.hs, interpreted
 )

obj/haskell2/Proof_default.hs:177:0:
    Functional dependencies conflict between instance declarations:
      instance [incoherent] SubstRep
                              (Prop (Sat VarName)) ([VarName], [Expr]) (Prop (Sa
t Expr))
        -- Defined at obj/haskell2/Proof_default.hs:177:0-75
      instance [incoherent] SubstRep
                              (Prop (Sat Int)) ([Int], [Expr]) (Prop (Sat Expr))

        -- Defined at obj/haskell2/Proof_default.hs:180:0-67
      instance [incoherent] SubstRep
                              (Prop (Sat Int)) ([Int], [Expr]) (Prop (Sat Expr))

        -- Defined at obj/haskell2/Proof_default.hs:180:0-67
Failed, modules loaded: none.

It has listed three items that conflict, but two of the items are actually the same. I would expect the error message to nub the list of instances first. Sample file attached. This bug may have been fixed in HEAD, with the recent warnings patch, but I cannot check (not enough disk space!).

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown