Re: [GHC] #15290: QuantifiedConstraints: panic "addTcEvBind NoEvBindsVar"

21 Jun 2018

      #15290: QuantifiedConstraints: panic "addTcEvBind NoEvBindsVar"
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:  9123, 14883
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Hm, I've hit a roadblock when trying to switch over to the scheme proposed
 in comment:5. Consider this code:

 {{{#!hs
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE ImpredicativeTypes #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
 {-# OPTIONS_GHC -ddump-deriv #-}
 module Bug where

 import Data.Coerce

 class C a where
   c :: Int -> forall b. b -> a

 instance C Int where
   c _ _ = 42

 newtype Age = MkAge Int

 -- This is what
 --
 --   deriving instance C Age
 --
 -- would generate:
 instance C Age where
   c = coerce @(             Int -> forall b. b -> a)
              c :: forall a. Int -> forall b. b -> a
 }}}

 This fails to typecheck:

 {{{
 $ /opt/ghc/8.4.3/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:26:7: error:
     • Couldn't match representation of type ‘forall b2. b2 -> a’
                                with that of ‘b1 -> a’
         arising from a use of ‘coerce’
     • In the expression:
           coerce @(Int -> forall b. b -> a) c ::
             forall a. Int -> forall b. b -> a
       In an equation for ‘c’:
           c = coerce @(Int -> forall b. b -> a) c ::
                 forall a. Int -> forall b. b -> a
       In the instance declaration for ‘C Age’
    |
 26 |   c = coerce @(             Int -> forall b. b -> a)
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
 }}}

 The same error occurs if I use `InstanceSigs`.

 Any ideas on how to make this work?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15290#comment:15
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