Arbitrary self types v2: add Deref test.

This case (found in IndexVec/IndexSlice within rustc_index) causes
unusual problems for deshadowing algorithms which we might want to add
in the Arbitrary Self Types v2 additions.

Author: adetaylor

tests/ui/self/conflicting_inner.rs

@@ -0,0 +1,38 @@
+//@ run-pass
+//@ revisions: default feature
+#![cfg_attr(feature, feature(arbitrary_self_types))]
+
+// This test aims to be like the IndexVec within rustc, and conflicts
+// over its into_iter().
+
+#[allow(dead_code)]
+trait Foo {
+    fn foo(self) -> usize;
+}
+
+struct IndexVec<T>(T);
+
+impl<T> std::ops::Deref for IndexVec<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<'a, T> Foo for &'a IndexVec<T> {
+    fn foo(self) -> usize {
+        2
+    }
+}
+
+impl<T> IndexVec<T> {
+    fn foo(self) -> usize {
+        1
+    }
+}
+
+fn main() {
+    let ivec = IndexVec(0usize);
+    assert_eq!(ivec.foo(), 1);
+}