TypeError is reported as "redundant constraint" starting with GHC 8.2
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 | |
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| Blocking | |
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| Operating system | |
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