Missing warning about redundant import for a class instance

I have the following module

module RedundantImport where

-- this import is redundant, but GHC does not spot it
import qualified Foreign.Storable as St

import Foreign.Storable (Storable, sizeOf, alignment, peek, poke, )
import Foreign.Ptr (castPtr, )


newtype T a = Cons a


instance Storable a => Storable (T a) where
   sizeOf (Cons a) = sizeOf a
   alignment (Cons a) = alignment a
   peek = fmap Cons . peek . castPtr
   poke p (Cons a) = poke (castPtr p) a

If loaded into GHCi with -Wall option, then GHCi does not give a warning, although the first import is redundant. It might have to do with the way how GHC accepts unqualified method names in instance implementations although the concerned method names are only imported in a qualified way.

Trac metadata

Trac field	Value
Version	6.12.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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CC	ghc@henning-thielemann.de
Operating system
Architecture