add auto traits regression test

Author: lcnr

src/test/ui/auto-traits/negative-impl-ambiguity.rs

@@ -0,0 +1,18 @@
+// check-pass
+use std::rc::Rc;
+
+trait A {
+    fn foo(&self) {}
+}
+
+impl<T: Send> A for T {}
+
+fn test<T: A>(rc: &Rc<T>) {
+    rc.foo()
+    // `Rc: Send` must not be evaluated as ambiguous
+    // for this to compile, as we are otherwise
+    // not allowed to use auto deref here to use
+    // the `T: A` implementation.
+}
+
+fn main() {}