[GHC] #11462: Use of typechecker plugin erroneously triggers "unbound implicit parameter" error

19 Jan 2016

      #11462: Use of typechecker plugin erroneously triggers "unbound implicit parameter"
error
-------------------------------------+-------------------------------------
           Reporter:  kwf            |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  7.10.3
  (Type checker)                     |
           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:
-------------------------------------+-------------------------------------
 To document this bug, we're going to need a typechecker plugin to test it
 with. I've built a dummy plugin for this purpose, so we can be sure it is
 not interference from a particular plugin.

 {{{dummy-plugin/dummy-plugin.cabal}}}
 {{{
 name:                dummy-plugin
 version:             0.1.0.0
 category:            Development
 build-type:          Simple
 cabal-version:       >=1.10

 library
   hs-source-dirs:      .
   exposed-modules:     DummyPlugin
   build-depends:       base, ghc
   default-language:    Haskell2010
   GHC-options:         -Wall -O2
 }}}

 {{{dummy-plugin/DummyPlugin.hs}}}
 {{{#!hs
 module DummyPlugin(plugin) where

 import TcRnMonad ( TcPlugin(..), TcPluginResult(..) )
 import Plugins ( defaultPlugin, Plugin(..), CommandLineOption )

 plugin :: Plugin
 plugin = defaultPlugin { tcPlugin = Just . thePlugin }

 thePlugin :: [CommandLineOption] -> TcPlugin
 thePlugin opts = TcPlugin
   { tcPluginInit  = return ()
   , tcPluginSolve = \_ _ _ _ -> return $ TcPluginOk [] []
   , tcPluginStop  = \_ -> return ()
   }
 }}}

 {{{Bug.hs}}}
 {{{#!hs
 {-# OPTIONS_GHC -fplugin=DummyPlugin #-}

 module Bug where

 impossible :: a
 impossible = undefined
 }}}

 First, compile the dummy plugin. From its directory, run {{{cabal
 install}}} to install the plugin.

 Then, from the main directory, run {{{ghc Bug.hs}}}.

 Expected result: the file compiles.
 Actual result:
 {{{
 Bug.hs:6:14: error:
     • Unbound implicit parameter ?callStack::GHC.Stack.Types.CallStack
         arising from a use of implicit parameter ‘?callStack’
     • In the expression: undefined
       In an equation for ‘impossible’: impossible = undefined
 }}}

 Further, observe that commenting out the line which invokes the type-
 checker plugin (the pragma on line 1) causes the file to compile
 correctly.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11462
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