[GHC] #13333: Typeable regression in GHC HEAD

[GHC]

#13333: Typeable regression in GHC HEAD
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I've observed something even stranger about this program. Let's tweak it
 slightly and add a couple more things (note that the definition of
 `dataCast1T` is unchanged):

 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 module DataCast where

 import Data.Data
 import GHC.Exts (Constraint)

 data T (phantom :: k) = T

 data D (p :: k -> Constraint) (x :: j) where
   D :: forall (p :: k -> Constraint) (x :: k). p x => D p x

 class Possibly p x where
   possibly :: proxy1 p -> proxy2 x -> Maybe (D p x)

 instance Possibly Data () where
   possibly _ _ = Just D

 dataCast1T :: forall k c t (phantom :: k).
               (Typeable k, Typeable t, Typeable phantom, Possibly Data
 phantom)
            => (forall d. Data d => c (t d))
            -> Maybe (c (T phantom))
 dataCast1T f = case possibly (Proxy :: Proxy Data) (Proxy :: Proxy
 phantom) of
                  Nothing -> Nothing
                  Just D  -> gcast1 f

 newtype Q q x = Q { unQ :: x -> q }
 ext1Q :: (Typeable k, Typeable t, Typeable (phantom :: k), Possibly Data
 phantom)
       => (T phantom -> q)
       -> (forall e. Data e => t e -> q)
       -> T phantom -> q
 ext1Q def ext arg =
   case dataCast1T (Q ext) of
     Just (Q ext') -> ext' arg
     Nothing       -> def arg
 }}}

 With GHC HEAD, this gives the same error as the original program. But if
 you add this one definition to this file:

 {{{#!hs
 test1 :: Char
 test1 = (const 'p') `ext1Q` (\T -> 'q') $ (T :: T ())
 }}}

 Then it typechecks without issue on GHC HEAD! This is pretty bizarre,
 considering that the error was originally reported in `dataCast1T`, and we
 managed to make the error go away without making a single change to
 `dataCast1T`.

 FWIW, this program also compiles without issue on GHC 8.0.2 (with and
 without `test1`).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13333#comment:2>
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