[GHC] #14218: GHC.Stack.HasCallStack not compatible with ConstraintKinds

[GHC]

#14218: GHC.Stack.HasCallStack not compatible with ConstraintKinds
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           Reporter:  ntc2           |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The programs `Good` and `Bad` are the same, except that `Bad` uses a
 constraint synonym including `GHC.Stack.HasCallStack` whereas `Good`
 inlines the constraints. I expect them to produce the same output, but
 they don't:
 {{{
 $ ./Good
 ["callStack","f"]

 $ ./Bad
 ["f"]
 }}}

 Here is the source for `Good.hs`:
 {{{#!hs
 {-# LANGUAGE MultiParamTypeClasses #-} -- For nullary 'Trivial' class
 module Main where

 import qualified GHC.Stack as Ghc

 class Trivial where
 instance Trivial where

 -- | Print the functions on the call stack.
 callStack :: (Ghc.HasCallStack, Trivial) => IO ()
 callStack = print $ map fst (Ghc.getCallStack Ghc.callStack)

 f :: (Ghc.HasCallStack, Trivial) => IO ()
 f = callStack

 main :: IO ()
 main = f -- Should print @["callStack", "f"]@.
 }}}

 Here is the source for `Bad.hs`:
 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}       -- For 'C'
 {-# LANGUAGE MultiParamTypeClasses #-} -- For nullary 'Trivial' class
 module Main where

 import qualified GHC.Stack as Ghc

 class Trivial where
 instance Trivial where

 type C = (Ghc.HasCallStack, Trivial)

 -- | Print the functions on the call stack.
 callStack :: C => IO ()
 callStack = print $ map fst (Ghc.getCallStack Ghc.callStack)

 f :: C => IO ()
 f = callStack

 main :: IO ()
 main = f -- Should print @["callStack", "f"]@.
 }}}

 Tested compiled and interpreted with GHC 8.2.1 and 8.0.2.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14218>
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