Add test for #28514

Fixes #28514

Author: petrochenkov

src/test/compile-fail/issue-28514.rs

@@ -0,0 +1,45 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+pub use inner::C;
+
+mod inner {
+    trait A {
+        fn a(&self) { }
+    }
+
+    pub trait B {
+        fn b(&self) { }
+    }
+
+    pub trait C: A + B { //~ ERROR private trait in public interface
+                         //~^ WARN will become a hard error
+        fn c(&self) { }
+    }
+
+    impl A for i32 {}
+    impl B for i32 {}
+    impl C for i32 {}
+
+}
+
+fn main() {
+    // A is private
+    // B is pub, not reexported
+    // C : A + B is pub, reexported
+
+    // 0.a(); // can't call
+    // 0.b(); // can't call
+    0.c(); // ok
+
+    C::a(&0); // can call
+    C::b(&0); // can call
+    C::c(&0); // ok
+}