standalone deriving breaks module hiding

Foo.hs:

module Foo ( T, t ) where
data T = T
t = T

Bar.hs:

import Foo
deriving instance Eq T

Then in ghci...

$ ghci Bar.hs -XStandaloneDeriving
[1 of 2] Compiling Bar              ( Bar.hs, interpreted )
[2 of 2] Compiling Main             ( Baz.hs, interpreted )
Ok, modules loaded: Bar, Main.
*Main> t == t
True

There is no way that I could implement that instance Eq T manually in Bar.hs because Foo exports T abstractly.

I see no reason that standalone deriving should have any magic ability that manual instance declarations do not have. Indeed it breaks module abstraction.

Trac metadata

Trac field	Value
Version	6.8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Blocking
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Operating system	Unknown
Architecture	Unknown