GHC accepts multiple instances for the same type in different modules

as mentioned by Simon PJ in this thread:

http://www.haskell.org/pipermail/haskell/2008-June/020436.html

here is the example, spelled out:

module B0 where
class C a where c :: a -> String
data T = T deriving Show

module B1 where
import B0
instance C T where c _ = "B1"
b = c T

module B2 where
import B0
instance C T where c _ = "B2"
b = c T

module Main where
import B1
import B2
main = print (B1.b,B2.b)

this is accepted without flags or errors and prints ("B1","B2").

the language report, section 4.3.2 clearly states:

A type may not be declared as an instance of a particular class more than once in the program.

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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Operating system	Unknown
Architecture	Unknown