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

25 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:  quantified-
                                     |  constraints/T15290, T15290a
      Blocked By:                    |             Blocking:  9123, 14883
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Actually, it turns out that the approach in comment:26 isn't feasible. It
 breaks on the program in comment:15, as it generates the following code:

 {{{#!hs
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
 module T15290b where

 import Data.Coerce

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

 instance C Int where
   c _ _ = 42

 newtype Age = MkAge Int
   -- deriving C

 instance C Age where
   c = coerce (c :: Int -> forall b. b -> Int) :: Int -> forall b. b -> Age
 }}}
 {{{
 $ ghc Bug.hs
 [1 of 1] Compiling T15290b          ( Bug.hs, Bug.o )

 Bug.hs:19:7: error:
     • Couldn't match representation of type ‘b0’ with that of ‘b1’
         arising from a use of ‘coerce’
       ‘b1’ is a rigid type variable bound by
         a type expected by the context:
           Int -> forall b1. b1 -> Age
         at Bug.hs:19:50-74
     • In the expression:
           coerce (c :: Int -> forall b. b -> Int) ::
             Int -> forall b. b -> Age
       In an equation for ‘c’:
           c = coerce (c :: Int -> forall b. b -> Int) ::
                 Int -> forall b. b -> Age
       In the instance declaration for ‘C Age’
    |
 19 |   c = coerce (c :: Int -> forall b. b -> Int) :: Int -> forall b. b
 -> Age
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 Since this does typecheck with the `TypeApplications`-based approach, I'll
 go with that one.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15290#comment:29
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