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{-# LANGUAGE MultiParamTypeClasses, FlexibleContexts, FlexibleInstances, OverlappingInstances, UndecidableInstances #-}
-- Trac #1470
module Foo where
class Sat a
class Data ctx a
instance Sat (ctx Char) => Data ctx Char
instance (Sat (ctx [a]), Data ctx a) => Data ctx [a]
class Data FooD a => Foo a
data FooD a = FooD
instance Foo t => Sat (FooD t)
instance Data FooD a => Foo a
instance Foo a => Foo [a]
{-
Given: Foo a,
and its superclasses: Data FooD a
Want superclass: Data FooD [a]
by instance Data FooD [a]
want: Sat (FooD [a])
Data FooD a -- We have this
by instance Sat (FooD t)
want: Foo [a]
BUT THIS INSTANCE OVERLAPS
-}
instance Foo [Char]
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