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{-# OPTIONS_GHC -Wno-redundant-constraints -Wno-simplifiable-class-constraints #-}
{-# LANGUAGE UndecidableInstances, MultiParamTypeClasses,
FunctionalDependencies, FlexibleInstances, FlexibleContexts #-}
module T3108 where
-- Direct recursion terminates (typechecking-wise)
class C0 x
where
m0 :: x -> ()
m0 = const undefined
instance {-# OVERLAPPING #-} (C0 x, C0 y) => C0 (x,y)
instance {-# OVERLAPPING #-} C0 Bool
instance {-# OVERLAPPABLE #-} C0 (x,Bool) => C0 x
foo :: ()
foo = m0 (1::Int)
-- Indirect recursion does not terminate (typechecking-wise)
class C1 x
where
m1 :: x -> ()
m1 = const undefined
instance {-# OVERLAPPING #-} (C1 x, C1 y) => C1 (x,y)
instance {-# OVERLAPPING #-} C1 Bool
instance {-# OVERLAPPABLE #-} (C2 x y, C1 (y,Bool)) => C1 x
-- Weird test case: (C1 (y,Bool)) is simplifiable
class C2 x y | x -> y
instance C2 Int Int
-- It is this declaration that causes nontermination of typechecking.
bar :: ()
bar = m1 (1::Int)
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