Fix regression: account for trait methods in arg count mismatch error

src/librustc/traits/error_reporting.rs

@@ -831,6 +831,11 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                 span,
                 node: hir::ImplItemKind::Method(hir::MethodSig { ref decl, .. }, _),
                 ..
+            }) |
+            hir::map::NodeTraitItem(&hir::TraitItem {
+                span,
+                node: hir::TraitItemKind::Method(hir::MethodSig { ref decl, .. }, _),
+                ..
             }) => {
                 (self.tcx.sess.codemap().def_span(span), decl.inputs.iter()
                         .map(|arg| match arg.clone().into_inner().node {

src/test/ui/issue-47706-trait.rs

@@ -0,0 +1,16 @@
+// 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.
+
+trait T {
+    fn f(&self, _: ()) {
+        None::<()>.map(Self::f);
+    }
+    //~^^ ERROR function is expected to take a single 0-tuple as argument
+}

src/test/ui/issue-47706-trait.stderr

@@ -0,0 +1,12 @@
+error[E0601]: main function not found
+
+error[E0593]: function is expected to take a single 0-tuple as argument, but it takes 2 distinct arguments
+  --> $DIR/issue-47706-trait.rs:13:20
+   |
+12 |     fn f(&self, _: ()) {
+   |     ------------------ takes 2 distinct arguments
+13 |         None::<()>.map(Self::f);
+   |                    ^^^ expected function that takes a single 0-tuple as argument
+
+error: aborting due to 2 previous errors
+