Add test for #2472

Author: brson

src/test/auxiliary/issue_2472_b.rs

@@ -0,0 +1,14 @@
+
+enum S = ();
+
+impl S {
+    fn foo() { }
+}
+
+trait T {
+    fn bar();
+}
+
+impl S: T {
+    fn bar() { }
+}

src/test/run-pass/issue-2472.rs

@@ -0,0 +1,11 @@
+// aux-build:issue_2472_b.rs
+
+use issue_2472_b;
+
+import issue_2472_b::{S, T};
+
+fn main() {
+    let s = S(());
+    s.foo();
+    s.bar();
+}