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{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
FlexibleInstances #-}
module ShouldFail where
class Foo f a r | f a -> r where
foo::f->a->r
-- These instances are incompatible because we can unify
-- the first two parameters, though it's rather obscure:
-- p -> (a,b)
-- t -> (,) (a,a)
-- c -> (,) a
-- r -> s
--
-- So a constraint which would sow this up is
-- Foo ((Int,Int)->Int)
-- ((Int,Int), (Int,Int))
-- t
-- This matches both. Not easy to spot, and the error
-- message would be improved by giving the unifier, or
-- a witness.
instance Foo (p->s) (t p) (t s)
instance Foo ((a,b)->r) (c a,c b)(c r)
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