Re: [GHC] #14832: QuantifiedConstraints: Adding to the context causes failure

27 Feb 2018

      #14832: QuantifiedConstraints: Adding to the context causes failure
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 I'm referring to the original program itself:

 {{{#!hs
 {-# Language QuantifiedConstraints, RankNTypes, PolyKinds,
 ConstraintKinds, UndecidableInstances, GADTs #-}

 import Control.Category
 import Data.Kind
 import Data.Coerce

 data With cls a b where
   With :: cls a b => With cls a b

 type Coercion = With Coercible

 type Refl  rel = (forall xx.       rel xx xx :: Constraint)
 type Trans rel = (forall xx yy zz. (rel xx yy, rel yy zz) => rel xx zz ::
 Constraint)

 instance (Refl rel, Trans rel) => Category (With rel) where
   id = With
   (.) = undefined
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14832#comment:7
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler