Currently, GHC's warning generation code is assuming that a name(RdrName) can
be imported at most once. This is a correct assumption, because 1) it's OK to
import same names as long as we don't use any of them 2) when we use one of
them, GHC generates an error because it doesn't disambiguate it automatically.
But apparently the story is different with typeclass methods. If I import two
methods with same names, it's OK to use them in typeclass instance
declarations, because the context specifies which one to use. For example, this
is OK (where modules A and B define typeclasses A and B, both with a
import A import B data Blah = Blah instance A Blah where has = Blah instance B Blah where has = Blah
But GHC's warning generator is not taking this into account, and so if I change
import list of this program to:
import A (A (has)) import B (B (has))
GHC is printing these warnings:
Main.hs:5:1: Warning: The import of ‘A.has’ from module ‘A’ is redundant Main.hs:6:1: Warning: The import of ‘B.has’ from module ‘B’ is redundant
Why? Because warning generation code is _silently_ ignoring multiple symbols
with same names.
With this patch, GHC takes this into account. If there's only one name, then
this patch reduces to the previous version, that is, it works exactly the same
as current GHC(thanks goes to @quchen for realizing this).