id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
5316,Orphan instances strike again: ghc rejects a program at first but will accept it if you repeat the same compilation command,jcpetruzza,,"Consider these two modules (boiled down example from the checkers package):

{{{
{-# LANGUAGE ScopedTypeVariables, FlexibleContexts #-}

module T1

where

import Test.QuickCheck
import Text.Show.Functions ()


f :: forall m a b.  ( Arbitrary (a->b) ) => m (a,b) -> Property
f = const $ property (undefined :: (a->b) -> Bool)
}}}

{{{
module T2  where

import Control.Concurrent

g = threadDelay maxBound
}}}

I see the following interaction:

{{{
$ rm *hi *.o
$ ghc --make -c -O  T1 T2
[1 of 2] Compiling T2               ( T2.hs, T2.o )
[2 of 2] Compiling T1               ( T1.hs, T1.o )

T1.hs:12:13:
    Overlapping instances for Show (a -> b)
      arising from a use of `property'
    Matching instances:
      instance Show (a -> b) -- Defined in Text.Show.Functions
      instance Show base:System.Event.Manager.IOCallback
        -- Defined in base:System.Event.Manager
    (The choice depends on the instantiation of `a, b'
     To pick the first instance above, use -XIncoherentInstances
     when compiling the other instance declarations)
    In the second argument of `($)', namely
      `property (undefined :: (a -> b) -> Bool)'
    In the expression: const $ property (undefined :: (a -> b) -> Bool)
    In an equation for `f':
        f = const $ property (undefined :: (a -> b) -> Bool)
$ ghc --make -c -O  T1 T2
[2 of 2] Compiling T1               ( T1.hs, T1.o )
$ ghc --make -c -O  T1 T2
$ ls
T1.hi T1.hs T1.o  T2.hi T2.hs T2.o
}}}

I see this consistent behaviour in versions 7.0.{1,2,3,4} but not with 6.12.1
",bug,new,low,_|_,Compiler,7.0.4,,,conal@…,Unknown/Multiple,Unknown/Multiple,GHC rejects valid program,,,,,