Add test for #30123

src/test/auxiliary/issue_30123_aux.rs

@@ -0,0 +1,33 @@
+// Copyright 2015 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.
+
+use std::marker::PhantomData;
+
+pub struct Directed;
+pub struct Undirected;
+
+pub struct Graph<N, E, Ty = Directed> {
+    nodes: Vec<PhantomData<N>>,
+    edges: Vec<PhantomData<E>>,
+    ty: PhantomData<Ty>,
+}
+
+
+impl<N, E> Graph<N, E, Directed> {
+    pub fn new() -> Self {
+        Graph{nodes: Vec::new(), edges: Vec::new(), ty: PhantomData}
+    }
+}
+
+impl<N, E> Graph<N, E, Undirected> {
+    pub fn new_undirected() -> Self {
+        Graph{nodes: Vec::new(), edges: Vec::new(), ty: PhantomData}
+    }
+}

src/test/compile-fail/issue-30123.rs

@@ -0,0 +1,19 @@
+// Copyright 2015 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.
+
+// aux-build:issue_30123_aux.rs
+
+extern crate issue_30123_aux;
+use issue_30123_aux::*;
+
+fn main() {
+    let ug = Graph::<i32, i32>::new_undirected();
+    //~^ ERR no associated item named `new_undirected` found for type
+}