Replace consider_unification_despite_ambiguity with obligation variant

src/test/compile-fail/closure-wrong-kind.rs

@@ -0,0 +1,23 @@
+// 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.
+
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+struct X;
+fn foo<T>(_: T) {}
+fn bar<T: Fn(u32)>(_: T) {}
+
+fn main() {
+    let x = X;
+    let closure = |_| foo(x);
+    //~^ ERROR the closure implements `FnOnce` but not `Fn`
+    bar(closure);
+}