unjustified overlapping instances error

When compiling hets (http://trac.informatik.uni-bremen.de:8080/hets/wiki/HetsForDevelopers) using ghc-7.6.0.20120810 I get the following error

[489 of 664] Compiling Comorphisms.HasCASL2IsabelleHOL ( Comorphisms/HasCASL2IsabelleHOL.hs, Comorphisms/HasCASL2IsabelleHOL.o )

Comorphisms/HasCASL2IsabelleHOL.hs:61:44:
    Overlapping instances for Category Env Morphism
      arising from a use of `sourceSublogic'
    Matching instances:
      instance [incoherent] Category Env Morphism
        -- Defined at HasCASL/Logic_HasCASL.hs:74:10
    There exists a (perhaps superclass) match:
    (The choice depends on the instantiation of `'
     To pick the first instance above, use -XIncoherentInstances
     when compiling the other instance declarations)
    In the second argument of `isSubElem', namely `sourceSublogic cid'
    In the expression: sl `isSubElem` sourceSublogic cid
    In the expression:
      if sl `isSubElem` sourceSublogic cid then Just () else Nothing

Note, that only a single instance is listed and that the choice is empty. Adding -XIncoherentInstances does also not help to get it compiled.

Trac metadata

Trac field	Value
Version	7.6.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
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Blocking
CC
Operating system	Solaris
Architecture	x86