Parser: Global single-segment paths should be represented as PatEnum

Fixed #15774.

src/libsyntax/parse/parser.rs

@@ -3101,7 +3101,7 @@ pub fn parse_pat(&mut self) -> Gc<Pat> {
                             }
                           },
                           _ => {
-                              if enum_path.segments.len() == 1 {
+                              if !enum_path.global && enum_path.segments.len() == 1 {
                                   // it could still be either an enum
                                   // or an identifier pattern, resolve
                                   // will sort it out:

src/test/run-pass/issue-15774.rs

@@ -0,0 +1,32 @@
+// Copyright 2014 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.
+
+#![deny(warnings)]
+#![allow(unused_imports)]
+
+enum Foo { A }
+mod bar {
+    pub fn normal(x: ::Foo) {
+        use A;
+        match x {
+            A => {}
+        }
+    }
+    pub fn wrong(x: ::Foo) {
+        match x {
+            ::A => {}
+        }
+    }
+}
+
+pub fn main() {
+    bar::normal(A);
+    bar::wrong(A);
+}