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TypeError is reported as "redundant constraint" starting with GHC 8.2

    • Closed Issue created by fsoikin @trac-fsoikin

    The following program compiles fine on GHC 8.0.2, but GHC 8.2.2 issues a warning, complaining that TypeError is a redundant constraint. This makes it very inconvenient to use TypeError with type classes.

    {-# OPTIONS_GHC -Wredundant-constraints -Wall -Werror #-}
import GHC.TypeLits (TypeError, ErrorMessage(..))

class C a where f :: a -> a
instance {-# OVERLAPPING #-} C Int where f _ = 42
instance {-# OVERLAPPABLE #-} TypeError ( 'Text "Only Int is supported" ) => C a where f = undefined

main :: IO ()
main = print $ f (42::Int)
    Trac metadata
    Trac field Value
    Version 8.2.2
    Type Bug
    TypeOfFailure OtherFailure
    Priority normal
    Resolution Unresolved
    Component Compiler
    Test case
    Differential revisions
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    Related
    Blocking
    CC
    Operating system
    Architecture
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