module C where

{-
This import is necessary but is reported as unnecessary.
"necessary" means:
If you remove that import statement, then the module cannot be compiled.
Of course, we could use the identifiers from B,
since B just re-exports A.
But formally I consider it a bug to declare the import of A as unnecessary.
-}
import qualified A

{-
This import is unnecessary but no warning is emitted.
If this import is removed, everything compiles fine without warnings.
-}
import B


f :: String -> A.T
f = read