Rollup merge of #23013 - nikomatsakis:syncbox, r=pnkfelix

When generating WF criteria, do not visit the same type more than once. Fixes an infinite stack overflow (#23003).

r? @aturon

src/test/compile-fail/traits-issue-23003-overflow.rs

@@ -0,0 +1,38 @@
+// Copyright 2015 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.
+
+// A variant of traits-issue-23003 in which an infinite series of
+// types are required. This currently creates an overflow. This test
+// is included to ensure that some controlled failure, at least,
+// results -- but it might be that we should adjust the rules somewhat
+// to make this legal. -nmatsakis
+
+use std::marker::PhantomData;
+
+trait Async {
+    type Cancel;
+}
+
+struct Receipt<A:Async> {
+    marker: PhantomData<A>,
+}
+
+struct Complete<B> {
+    core: Option<B>,
+}
+
+impl<B> Async for Complete<B> {
+    type Cancel = Receipt<Complete<Option<B>>>;
+}
+
+fn foo(r: Receipt<Complete<()>>) { }
+//~^ ERROR overflow
+
+fn main() { }