Add test for issue 3305

Author: catamorphism

src/test/run-pass/issue-3305.rs

@@ -0,0 +1,16 @@
+trait double {
+    fn double() -> uint;
+}
+
+impl uint: double {
+    fn double() -> uint { self * 2u }
+}
+
+fn is_equal<D: double>(x: @D, exp: uint) {
+    assert x.double() == exp;
+}
+
+fn main() {
+    let x = @(3u as double);
+    is_equal(x, 6);
+}