Technically in Haskell 2010 this is illegal:

module A( f ) where
  f = True

module B where
  import A hiding( g )  -- A does not export g
  g = f

The import A hiding( g ) in module B is technically an error (Haskell Report, 5.3.1) because A does not export g. However GHC allows it, in the interests of supporting backward compatibility; for example, a newer version of A might export g, and you want B to work in either case.

The warning -Wdodgy-imports, which is off by default but included with -W, warns if you hide something that the imported module does not export.