overlapping instances in 6.6 candidate

[Serge D. Mechveliani]

Dear GHC developers,

Can you, please, explain what has happened in  6.5.20060831
with treating of  overlapping instances?

It seems to understand them differently than  ghc-6.4.1.
For example, I compile the module

------------------------
class C a where c :: a -> Bool

instance C Int where c n = n > 0

instance C a => C [a] where  c xs =  and $ map c xs

instance C [Int] where  c xs =  c $ head xs

main = putStr (shows (c [1, (-2 :: Int)]) "\n")

instance Show [Bool] where  showsPrec _ xs =  shows (head xs)
------------------------ 

with the options
$dmCpOpt =
  -fglasgow-exts -fallow-undecidable-instances 
  -fallow-overlapping-instances -fno-warn-overlapping-patterns 
  -fwarn-unused-binds -fwarn-unused-matches -fwarn-unused-imports
:
> ghc $dmCpOpt --make Main

It reports

--------------------------------------------------------
[1 of 1] Compiling Main             ( Main.hs, Main.o )

Main.hs:11:0:
    Overlapping instances for Show [Bool]
      arising from use of `GHC.Show.$dmshowList' at Main.hs:11:0
    Matching instances:
      instance (Show a) => Show [a] -- Defined in GHC.Show
      instance [overlap ok] Show [Bool] -- Defined at Main.hs:11:0
    In the expression: GHC.Show.$dmshowList
    In the definition of `showList': showList = GHC.Show.$dmshowList
    In the definition for method `showList'
--------------------------------------------

And it takes as all right the overlaps for the class  C.

[Bool]  is a substitutional instance of  [a],  and according to the GHC 
specification, the Show instance for [Bool] must override the more generic
one for  Show a => [a].

Thank you in advance for explanation.

-----------------
Serge Mechveliani
mechvel at botik.ru