Normalize return type when checking for E0269

Fixes #31597

Author: jonas-schievink

src/librustc/middle/liveness.rs

@@ -113,6 +113,8 @@ use dep_graph::DepNode;
 use middle::def::*;
 use middle::pat_util;
 use middle::ty::{self, TyCtxt};
+use middle::traits;
+use middle::infer;
 use lint;
 use util::nodemap::NodeMap;
 
@@ -1490,9 +1492,17 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
 
         match fn_ret {
             ty::FnConverging(t_ret)
-                if self.live_on_entry(entry_ln, self.s.no_ret_var).is_some() => {
+                    if self.live_on_entry(entry_ln, self.s.no_ret_var).is_some() => {
 
-                if t_ret.is_nil() {
+                let infcx = infer::new_infer_ctxt(&self.ir.tcx, &self.ir.tcx.tables, None);
+                let mut selcx = traits::SelectionContext::new(&infcx);
+                let cause = traits::ObligationCause::dummy();
+
+                let norm = traits::normalize(&mut selcx,
+                                             cause,
+                                             &t_ret);
+
+                if norm.value.is_nil() {
                     // for nil return types, it is ok to not return a value expl.
                 } else {
                     let ends_with_stmt = match body.expr {

src/test/run-pass/issue-31597.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.
+
+trait Make {
+    type Out;
+
+    fn make() -> Self::Out;
+}
+
+impl Make for () {
+    type Out = ();
+
+    fn make() -> Self::Out {}
+}
+
+fn main() {}