resolve type variables in the custom type op pathway

src/librustc/traits/query/type_op/custom.rs

@@ -106,7 +106,8 @@ fn scrape_region_constraints<'gcx, 'tcx, R>(
         infcx.tcx,
         region_obligations
             .iter()
-            .map(|(_, r_o)| (r_o.sup_type, r_o.sub_region)),
+            .map(|(_, r_o)| (r_o.sup_type, r_o.sub_region))
+            .map(|(ty, r)| (infcx.resolve_type_vars_if_possible(&ty), r)),
         &region_constraint_data,
     );
 

src/test/ui/issue-53568.rs

@@ -0,0 +1,61 @@
+// Copyright 2018 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 an NLL-related ICE (#53568) -- we failed to
+// resolve inference variables in "custom type-ops".
+//
+// compile-pass
+
+#![feature(nll)]
+#![allow(dead_code)]
+
+trait Future {
+    type Item;
+}
+
+impl<F, T> Future for F
+where F: Fn() -> T
+{
+    type Item = T;
+}
+
+trait Connect {}
+
+struct Connector<H> {
+    handler: H,
+}
+
+impl<H, T> Connect for Connector<H>
+where
+    T: 'static,
+    H: Future<Item = T>
+{
+}
+
+struct Client<C> {
+    connector: C,
+}
+
+fn build<C>(_connector: C) -> Client<C> {
+    unimplemented!()
+}
+
+fn client<H>(handler: H) -> Client<impl Connect>
+where H: Fn() + Copy
+{
+    let connector = Connector {
+        handler,
+    };
+    let client = build(connector);
+    client
+}
+
+fn main() { }
+