False "duplicate or overlapping instances" message

[George Russell]

When I compile the attached file on Linux with 4.08.1:
   /home/ger/ghc-4.08.1-binary/bin/ghc -c Instances.hs -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances
I get the message:

Instances.hs:1:
    Duplicate or overlapping instance declarations
        for `HasConfig (arcTypeConfig value) (arcTypeParms value)'
            defined at Instances.hs:19 and defined at Instances.hs:11

This is not fair, because while the instances for HasConfig have the
potential to conflict in the future, they will only do so should the classes
NodeTypeConfigParms and ArcTypeConfigParms overlap.
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