Use of typechecker plugin erroneously triggers "unbound implicit parameter" error

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

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

{-# 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.

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed milestone to %8.0.1

changed weight to 7

added Tbug Trac import typechecker labels

changed weight to 10

Thank you for reducing the problem to this simple testcase.

This is a regression from ghc-7.10.3.

Trac metadata

Trac field	Value
Version	7.10.3 → 8.0.1-rc1
Priority	high → highest

assigned to @gridaphobe

Trac metadata

Trac field	Value
Test case	- → T11462
Differential revisions	- → D1804

mentioned in commit 835a2a24

closed

Merged to ghc-8.0 in 38dc961b.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added plugins label

added typechecker plugins label and removed typechecker label

added Phighest label