[tests] Add passing test

Closes #3683

Author: catamorphism

src/test/run-pass/issue-3683.rs

@@ -0,0 +1,16 @@
+trait Foo {
+    fn a() -> int;
+    fn b() -> int {
+        self.a() + 2
+    }
+}
+
+impl int: Foo {
+    fn a() -> int {
+        3
+    }
+}
+
+fn main() {
+    assert(3.b() == 5);
+}