Fix: added missing guard.

com.oracle.truffle.r.nodes.builtin/src/com/oracle/truffle/r/nodes/builtin/base/fastpaths/IsElementFastPath.java

@@ -57,7 +57,7 @@ public abstract class IsElementFastPath extends RFastPathNode {
         return RRuntime.LOGICAL_FALSE;
     }
 
-    @Specialization(replaces = "iselementOneCachedString")
+    @Specialization(guards = "elIn.getLength() == 1", replaces = "iselementOneCachedString")
     protected Byte iselementOne(RAbstractStringVector elIn, RAbstractStringVector setIn,
                     @Cached("create()") BranchProfile trueProfile,
                     @Cached("create()") BranchProfile falseProfile) {

com.oracle.truffle.r.test/src/com/oracle/truffle/r/test/library/base/TestIsElement.java

+/*
+ * Copyright (c) 2017, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+package com.oracle.truffle.r.test.library.base;
+
+import org.junit.Test;
+
+import com.oracle.truffle.r.test.TestBase;
+
+public class TestIsElement extends TestBase {
+
+    @Test
+    public void testIsElement() {
+        assertEval("{ is.element(c('a', 'b', 'b'), 'b') }");
+        assertEval("{ is.element('b', c('a', 'b', 'b')) }");
+        assertEval("{ is.element(paste0('a', 1:10), 'a1') }");
+        assertEval("{ is.element(paste0('a', 1:10), 'a0') }");
+        assertEval("{ is.element(paste0('a', c(1:10, rep(11,2))), 'a11') }");
+        assertEval("{ is.element(c(1:10,rep(11,10)), 1) }");
+        assertEval("{ is.element(c(1:10,rep(11,10)), 11) }");
+        assertEval("{ is.element(c(1:10,rep(11,10)), c(1:10,rep(11,10))) }");
+    }
+}