-fallow-incoherent-instances

[Christian Maeder]

The attached module does not compile and yields the following error:

InCoherentInst.hs:17:
     Could not deduce (Confuse a) from the context (Typeable a)
       arising from use of `breakFn' at InCoherentInst.hs:17
     Probable fix:
         Add (Confuse a) to the type signature(s) for `addGeneralFallOut'
     In the first argument of `GeneralBreakFn', namely `breakFn'
     In the definition of `addGeneralFallOut':
         addGeneralFallOut = let
                               breakFn a = throwDyn (GeneralFallOutExcep a)
                             in GeneralBreakFn breakFn


The same source compiles ok without -fallow-incoherent-instances (or 
with -fno-allow-incoherent-instances).

If, furthermore, the "confusing instance" is commented out, the source 
even compiles without extensions.

I don't know if this is a bug, possibly related to the import of 
Typeable stuff. I don't need a fix. I only want to point out that 
globally switching on the option -fallow-incoherent-instances is likely 
to break existing code, currently (ghc 6.2.2).

Cheers Christian

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