?

{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}

class GEq a b where
    geq :: a -> b -> Bool

instance {-# OVERLAPPABLE #-} (Eq a) => GEq a a where
    geq = (==)

instance {-# OVERLAPPING #-} GEq a b where
    geq _ _ = False

GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Prelude> :load GEq.hs 
[1 of 1] Compiling Main             ( GEq.hs, interpreted )
Ok, modules loaded: Main.
*Main> geq () ()

<interactive>:2:1: error:
    • Overlapping instances for GEq () () arising from a use of ‘geq’
      Matching instances:
        instance [overlapping] [safe] GEq a b -- Defined at GEq.hs:9:30
        instance [overlappable] [safe] Eq a => GEq a a
          -- Defined at GEq.hs:6:31
    • In the expression: geq () ()
      In an equation for ‘it’: it = geq () ()
*Main> 

Trac metadata

Trac field	Value
Version	8.0.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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