register fn-ptr coercion obligations out of a snapshot

Fixes #43923.

src/librustc_typeck/check/coercion.rs

@@ -807,8 +807,7 @@ fn try_find_coercion_lub<E>(&self,
             let lub_ty = self.commit_if_ok(|_| {
                 self.at(cause, self.param_env)
                     .lub(prev_ty, new_ty)
-                    .map(|ok| self.register_infer_ok_obligations(ok))
-            });
+            }).map(|ok| self.register_infer_ok_obligations(ok));
 
             if lub_ty.is_ok() {
                 // We have a LUB of prev_ty and new_ty, just return it.

src/test/run-pass/issue-43923.rs

+// Copyright 2017 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.
+
+struct A<T: ?Sized> { ptr: T }
+
+fn foo<T>(x: &A<[T]>) {}
+
+fn main() {
+    let a = foo;
+    let b = A { ptr: [a, a, a] };
+    a(&A { ptr: [()] });
+}