rustc: Avoid out of bounds in check_match

Closes #12116

Author: alexcrichton

src/librustc/middle/check_match.rs

@@ -247,6 +247,7 @@ fn is_useful(cx: &MatchCheckCtxt, m: &matrix, v: &[@Pat]) -> useful {
                 _ => *r.get(0)
             }
         }
+        None if v.len() == 0 => return not_useful,
         None => v[0]
     };
     let left_ty = if real_pat.id == 0 { ty::mk_nil() }

src/test/compile-fail/issue-12116.rs

@@ -0,0 +1,25 @@
+// Copyright 2012-2014 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.
+
+enum IntList {
+    Cons(int, Box<IntList>),
+    Nil
+}
+
+fn tail(source_list: &IntList) -> IntList {
+    match source_list {
+        &Cons(val, box ref next_list) => tail(next_list),
+        &Cons(val, box Nil)           => Cons(val, box Nil),
+        //~^ ERROR: unreachable pattern
+        _                          => fail!()
+    }
+}
+
+fn main() {}