[GHC] #14832: QuantifiedConstraints: Strengthening context causes failure

21 Feb 2018

      #14832: QuantifiedConstraints: Strengthening context causes failure
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  Operating System:  Unknown/Multiple
  QuantifiedConstraints wipT2893     |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 With Simon's latest changes (ticket:14733#comment:9) this works:

 {{{#!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 => Category (With rel) where
   id = With
   (.) = undefined
 }}}

 But strengthening the context with `Trans rel`:

 {{{#!hs
 instance (Refl rel, Trans rel) => Category (With rel) where
 }}}

 causes it to fail

 {{{
 GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( J.hs, interpreted )

 J.hs:15:10: error:
     • Could not deduce: rel xx zz
       from the context: (Refl rel, Trans rel)
         bound by an instance declaration:
                    forall k (rel :: k -> k -> Constraint).
                    (Refl rel, Trans rel) =>
                    Category (With rel)
         at J.hs:15:10-53
       or from: (rel xx yy, rel yy zz)
         bound by a quantified context at J.hs:15:10-53
     • In the ambiguity check for an instance declaration
       To defer the ambiguity check to use sites, enable
 AllowAmbiguousTypes
       In the instance declaration for ‘Category (With rel)’
    |
 15 | instance (Refl rel, Trans rel) => Category (With rel) where
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 J.hs:15:10: error:
     • Could not deduce: rel xx xx
       from the context: (Refl rel, Trans rel)
         bound by an instance declaration:
                    forall k (rel :: k -> k -> Constraint).
                    (Refl rel, Trans rel) =>
                    Category (With rel)
         at J.hs:15:10-53
     • In the ambiguity check for an instance declaration
       To defer the ambiguity check to use sites, enable
 AllowAmbiguousTypes
       In the instance declaration for ‘Category (With rel)’
    |
 15 | instance (Refl rel, Trans rel) => Category (With rel) where
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Failed, no modules loaded.
 Prelude>
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14832
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