Re: [GHC] #14833: QuantifiedConstraints: GHC can't deduce (() :: Constraint)?

23 Feb 2018

      #14833: QuantifiedConstraints: GHC can't deduce (() :: Constraint)?
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints wipT2893
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Note that the `(:-)` part is not needed to trigger this:

 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE QuantifiedConstraints #-}
 {-# LANGUAGE UndecidableInstances #-}
 module Bug where

 data Dict c where
   Dict :: c => Dict c

 class    (a => b) => Implies a b
 instance (a => b) => Implies a b

 iota1 :: (() => a) => Dict a
 iota1 = Dict

 iota2 :: Implies () a => Dict a
 iota2 = Dict
 }}}

 Note that `iota1` typechecks but `iota2` does not.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14833#comment:1
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