add tests for reserved words. ban stores against _score

modules/lang-painless/src/main/java/org/elasticsearch/painless/AnalyzerExternal.java

@@ -448,7 +448,12 @@ class AnalyzerExternal {
                 throw new IllegalArgumentException(AnalyzerUtility.error(ctx) + "Unknown variable [" + id + "].");
             }
 
+            // special cases: reserved words
             if ("_score".equals(id)) {
+                // read-only: don't allow stores
+                if (parentemd.storeExpr != null) {
+                    throw new IllegalArgumentException(AnalyzerUtility.error(ctx) + "Variable [" + id + "] is read-only.");
+                }
                 metadata.scoreValueUsed = true;
             }
 

modules/lang-painless/src/test/java/org/elasticsearch/painless/ReservedWordTests.java

@@ -0,0 +1,40 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.painless;
+
+/** Tests for special reserved words such as _score */
+public class ReservedWordTests extends ScriptTestCase {
+    
+    /** check that we can't declare a variable of _score, its really reserved! */
+    public void testScoreVar() {
+        IllegalArgumentException expected = expectThrows(IllegalArgumentException.class, () -> {
+            exec("int _score = 5; return _score;");
+        });
+        assertTrue(expected.getMessage().contains("Variable name [_score] already defined"));
+    }
+    
+    /** check that we can't write to _score, its read-only! */
+    public void testScoreStore() {
+        IllegalArgumentException expected = expectThrows(IllegalArgumentException.class, () -> {
+            exec("_score = 5; return _score;");
+        });
+        assertTrue(expected.getMessage().contains("Variable [_score] is read-only"));
+    }
+}