add regression test

Fixes #47139

src/test/run-pass/issue-47139-1.rs

@@ -0,0 +1,87 @@
+// 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.
+
+// Regression test for issue #47139:
+//
+// Coherence was encountering an (unnecessary) overflow trying to
+// decide if the two impls of dummy overlap.
+//
+// The overflow went something like:
+//
+// - `&'a ?T: Insertable` ?
+// - let ?T = Option<?U> ?
+// - `Option<?U>: Insertable` ?
+// - `Option<&'a ?U>: Insertable` ?
+// - `&'a ?U: Insertable` ?
+//
+// While somewhere in the middle, a projection would occur, which
+// broke cycle detection.
+//
+// It turned out that this cycle was being kicked off due to some
+// extended diagnostic attempts in coherence, so removing those
+// sidestepped the issue for now.
+
+#![allow(dead_code)]
+
+pub trait Insertable {
+    type Values;
+
+    fn values(self) -> Self::Values;
+}
+
+impl<T> Insertable for Option<T>
+    where
+    T: Insertable,
+    T::Values: Default,
+{
+    type Values = T::Values;
+
+    fn values(self) -> Self::Values {
+        self.map(Insertable::values).unwrap_or_default()
+    }
+}
+
+impl<'a, T> Insertable for &'a Option<T>
+    where
+    Option<&'a T>: Insertable,
+{
+    type Values = <Option<&'a T> as Insertable>::Values;
+
+    fn values(self) -> Self::Values {
+        self.as_ref().values()
+    }
+}
+
+impl<'a, T> Insertable for &'a [T]
+{
+    type Values = Self;
+
+    fn values(self) -> Self::Values {
+        self
+    }
+}
+
+trait Unimplemented { }
+
+trait Dummy { }
+
+struct Foo<T> { t: T }
+
+impl<'a, U> Dummy for Foo<&'a U>
+    where &'a U: Insertable
+{
+}
+
+impl<T> Dummy for T
+    where T: Unimplemented
+{ }
+
+fn main() {
+}

src/test/run-pass/issue-47139-2.rs

@@ -0,0 +1,75 @@
+// 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.
+
+// Regression test for issue #47139:
+//
+// Same as issue-47139-1.rs, but the impls of dummy are in the
+// opposite order. This influenced the way that coherence ran and in
+// some cases caused the overflow to occur when it wouldn't otherwise.
+// In an effort to make the regr test more robust, I am including both
+// orderings.
+
+#![allow(dead_code)]
+
+pub trait Insertable {
+    type Values;
+
+    fn values(self) -> Self::Values;
+}
+
+impl<T> Insertable for Option<T>
+    where
+    T: Insertable,
+    T::Values: Default,
+{
+    type Values = T::Values;
+
+    fn values(self) -> Self::Values {
+        self.map(Insertable::values).unwrap_or_default()
+    }
+}
+
+impl<'a, T> Insertable for &'a Option<T>
+    where
+    Option<&'a T>: Insertable,
+{
+    type Values = <Option<&'a T> as Insertable>::Values;
+
+    fn values(self) -> Self::Values {
+        self.as_ref().values()
+    }
+}
+
+impl<'a, T> Insertable for &'a [T]
+{
+    type Values = Self;
+
+    fn values(self) -> Self::Values {
+        self
+    }
+}
+
+trait Unimplemented { }
+
+trait Dummy { }
+
+struct Foo<T> { t: T }
+
+impl<T> Dummy for T
+    where T: Unimplemented
+{ }
+
+impl<'a, U> Dummy for Foo<&'a U>
+    where &'a U: Insertable
+{
+}
+
+fn main() {
+}