explicit withFilter calls are being converted to filter calls

[scabug]

I understand that when compiling guards in for comprehensions, filter will be called if withFilter is not defined.

Unfortunately, it seems that explicit calls to withFilter are also being transformed to calls of filter.

          def empty = Iterator.range(0, 10).
                       withFilter(isEven _).
                       withFilter(isOdd _)

warning: `withFilter' method does not yet exist on Iterator,
using `filter' method instead

IMHO a compiler error should be reported instead.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-2700?orig=1
Reporter: Eric Willigers (ewilligers)

[scabug]

@odersky said:
I know it's not really the right thing to do, but:

1. This is a temporary measure. For Scala 2.8.1, the filter idiom will be deprecated
   in for comprehensions and for Scala 2.9 it will be removed.

2. The warning is not so bad.

So I don't think this warrants the effort spent to fix this (thsi effirt would be considerable; we'd have to re-rig the parser and type checker to treat for comprehensions as being different from their expansion.