More comments.

modules/lang-painless/src/main/java/org/elasticsearch/painless/node/EChain.java

@@ -269,14 +269,14 @@ public final class EChain extends AExpression {
      */
     @Override
     void write(MethodWriter writer) {
+        writer.writeDebugInfo(location);
+
         // For the case where the chain represents a String concatenation
-        // we must first write debug information, and then depending on the
-        // Java version write a StringBuilder or track types going onto the
-        // stack.  This must be done before the links in the chain are read
-        // because we need the StringBuilder to be placed on the stack
-        // ahead of any potential concatenation arguments.
+        // we must, depending on the Java version, write a StringBuilder or
+        // track types going onto the stack.  This must be done before the
+        // links in the chain are read because we need the StringBuilder to
+        // be placed on the stack ahead of any potential concatenation arguments.
         if (cat) {
-            writer.writeDebugInfo(location);
             writer.writeNewStrings();
         }
 

modules/lang-painless/src/main/java/org/elasticsearch/painless/node/SDefEach.java

@@ -0,0 +1,23 @@
+/*
+ * 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.node;
+
+public class SDefEach {
+}

modules/lang-painless/src/main/java/org/elasticsearch/painless/node/SEach.java

@@ -88,6 +88,9 @@ public class SEach extends AStatement {
             Type itr = Definition.getType("Iterator");
 
             variable = variables.addVariable(location, type, name, true, false);
+
+            // We must store the iterator as a variable for securing a slot on the stack, and
+            // also add the location offset to make the name unique in case of nested for each loops.
             iterator = variables.addVariable(location, itr, "#itr" + location.getOffset(), true, false);
 
             method = expression.actual.struct.methods.get(new MethodKey("iterator", 0));