Incoherent instance solving is over-eager

danilo2 writes (originally in #9432 #13284)

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE OverlappingInstances #-}
{-# LANGUAGE IncoherentInstances #-} -- the flag is niot needed by the example

module Main where
import Data.Typeable


class CTest a b | a -> b where
    cTest :: a -> b

-- this instance is choosen even if more specific is available!
instance out~a => CTest a out where
    cTest = id

instance CTest Int String where
    cTest _ = "test"


main = do
    print $ typeOf $ cTest (5::Int)

The instance CTest a out even if more specific (CTest Int String) is in scope, which just breaks how OverlappingInstances work. If we disable the IncoherentInstances flag, the right one is selected.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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