[GHC] #15232: TypeError is reported as "redundant constraint" starting with GHC 8.2

5 Jun 2018


      #15232: TypeError is reported as "redundant constraint" starting with GHC 8.2
-------------------------------------+-------------------------------------
           Reporter:  fsoikin        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.3
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
  CustomTypeErrors                   |
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 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.

 {{{#!hs
 {-# 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)
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15232
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