overlapping instances and separate compilation

There's three issues here that all seem to revolve around overlapping instances and separate compilation.

1. First, a documentation issue:

It is not clear whether it makes sense to set '-fallow-overlapping-instances' with different values per compilation unit or this flag should be constant across the whole build.

2. Secondly, I see that code that once worked fine in ghc 6.4 no longer compiles with ghc 6.5. Note these files:

Makefile:

main: Main.hs Indexed.o
	ghc -o main Main.hs Indexed.o

Indexed.o : Indexed.hs
	ghc -fglasgow-exts -c $<                                  # Fails
	#ghc -fglasgow-exts -fallow-overlapping-instances -c $<   # Works

Main.hs:

{-# OPTIONS -fglasgow-exts -fallow-overlapping-instances #-}
module Main where
import Indexed
    
instance Indexed [Int] Int where
   xs @@ i = xs !! fromIntegral i

f :: [Int] -> Int       
f ys = ys @@ (1 :: Int)

main = print (f [5..])

Indexed.hs:

module Indexed where

import Data.List

class Indexed f a where
    (@@) :: Integral i => f -> i -> a

instance Indexed [a] a where
    (@@) = genericIndex

I get the following:

$ make clean; make
rm *.hi *.o
ghc -fglasgow-exts -c Indexed.hs                                  # Fails
#ghc -fglasgow-exts -fallow-overlapping-instances -c Indexed.hs   # Works
ghc -o main Main.hs Indexed.o

Main.hs:9:7:
    Overlapping instances for Indexed [Int] Int
      arising from use of `@@' at Main.hs:9:7-22
    Matching instances:
      instance Indexed [a] a -- Imported from Indexed
      instance [overlap ok] Indexed [Int] Int -- Defined at Main.hs:5:0
    In the expression: ys @@ (1 :: Int)
    In the definition of `f': f ys = ys @@ (1 :: Int)
make: *** [main] Error 1

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.5.20060526

Is this a bug or a 6.5 feature? If it's a feature, then is there no way to have a single object file that can be used in one project that uses overlapping instances and another which doesn't?

3. Thirdly, I've noticed that --make (in ghc and implicitly in ghci) seems broken here. The recompilation checker doesn't seem to regenerate the .hi files when pragmas such as the following come and go in the file:

  {-# OPTIONS -fallow-overlapping-instances -#}

And it appears that in 6.5 the .hi should change here.

Trac metadata

Trac field	Value
Version	6.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown