[GHC] #8644: 'Untouchable' error with constraint variable in rank-2 type

[GHC]

#8644: 'Untouchable' error with constraint variable in rank-2 type
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       Reporter:  sbarclay           |             Owner:
           Type:  bug                |            Status:  new
       Priority:  normal             |         Milestone:
      Component:  Compiler (Type     |           Version:  7.7
  checker)                           |  Operating System:  Unknown/Multiple
       Keywords:                     |   Type of failure:  GHC rejects
   Architecture:  Unknown/Multiple   |  valid program
     Difficulty:  Unknown            |         Test Case:
     Blocked By:                     |          Blocking:
Related Tickets:                     |
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 The following program compiles fine with GHC 7.6.3:

 {{{#!haskell
 {-# LANGUAGE ConstraintKinds, RankNTypes, GADTs #-}

 data Dict c where Dict :: c => Dict c

 foo :: Dict c -> (c => r) -> r
 foo Dict x = x

 bar :: Dict ()
 bar = Dict

 main :: IO ()
 main = print $ foo bar "Hello"
 }}}

 However, it produces the following error with HEAD:

 {{{
 ConstraintIssue.hs:12:8:
     No instance for (Show s0) arising from a use of ‛print’
     The type variable ‛s0’ is ambiguous
     Note: there are several potential instances:
       instance Show Double -- Defined in ‛GHC.Float’
       instance Show Float -- Defined in ‛GHC.Float’
       instance (Integral a, Show a) => Show (GHC.Real.Ratio a)
         -- Defined in ‛GHC.Real’
       ...plus 24 others
     In the expression: print
     In the expression: print $ foo bar "Hello"
     In an equation for ‛main’: main = print $ foo bar "Hello"

 ConstraintIssue.hs:12:24:
     Couldn't match expected type ‛s0’ with actual type ‛[Char]’
       ‛s0’ is untouchable
         inside the constraints (())
         bound by a type expected by the context: (()) => s0
         at ConstraintIssue.hs:12:16-30
     In the second argument of ‛foo’, namely ‛"Hello"’
     In the second argument of ‛($)’, namely ‛foo bar "Hello"’
     In the expression: print $ foo bar "Hello"
 }}}

 If the type signature of 'main' is removed, the code compiles OK.

 If the empty constraint () is replaced with an equality constraint such as
 Int ~ Int, then the above error occurs with both 7.6.3 and HEAD, which I
 guess is intended behaviour.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8644>
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