This doesnt produce an error

Author: oberien

src/libcore/iter/mod.rs

@@ -1368,9 +1368,8 @@ impl<I: Iterator, P> Iterator for Filter<I, P> where P: FnMut(&I::Item) -> bool
     }
 
     #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        let (_, upper) = self.iter.size_hint();
-        (0, upper) // can't know a lower bound, due to the predicate
+    default fn size_hint(&self) -> (usize, Option<usize>) {
+        FilterImpl::size_hint(self)
     }
 
     // this special case allows the compiler to make `.filter(_).count()`
@@ -1418,6 +1417,39 @@ impl<I: Iterator, P> Iterator for Filter<I, P> where P: FnMut(&I::Item) -> bool
     }
 }
 
+use ops::RangeFrom;
+///
+#[unstable(feature="test", issue="0")]
+pub trait FancyIter: Iterator {}
+#[unstable(feature="test", issue="0")]
+impl<T: Step> FancyIter for RangeFrom<T> {}
+
+#[doc(hidden)]
+trait FilterImpl {
+    fn size_hint(&self) -> (usize, Option<usize>);
+}
+
+#[unstable(feature="test", issue="0")]
+impl<I: Iterator, P: FnMut(&I::Item) -> bool> FilterImpl for Filter<I, P> {
+    default fn size_hint(&self) -> (usize, Option<usize>) {
+        let (_, upper) = self.iter.size_hint();
+        (0, upper) // can't know a lower bound, due to the predicate
+    }
+}
+
+#[unstable(feature="test", issue="0")]
+impl<I: FancyIter, P: FnMut(&I::Item) -> bool> FilterImpl for Filter<I, P> {
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, None)
+    }
+}
+
+#[allow(warnings)]
+fn foo() {
+    let foo: isize = (0..).filter(|i| *i < 5).next().unwrap();
+}
+
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I: DoubleEndedIterator, P> DoubleEndedIterator for Filter<I, P>
     where P: FnMut(&I::Item) -> bool,