Do not cache ambiguous results unless there is at least some inference by-product within.

Fixes #19499.

src/test/run-pass/issue-19499.rs

@@ -0,0 +1,20 @@
+// 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.
+
+// Regression test for issue #19499. Due to incorrect caching of trait
+// results for closures with upvars whose types were not fully
+// computed, this rather bizarre little program (along with many more
+// reasonable examples) let to ambiguity errors about not being able
+// to infer sufficient type information.
+
+fn main() {
+    let n = 0;
+    let it = Some(1_us).into_iter().inspect(|_| {n;});
+}