Fix String concatentation bug.

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

@@ -262,8 +262,8 @@ public final class EAssignment extends AExpression {
 
             rhs.write(writer, globals); // write the bytecode for the rhs
 
-            if (!(rhs instanceof EBinary) || ((EBinary)rhs).cat) {
-                writer.writeAppendStrings(rhs.actual); // append the rhs's value unless it's also a concatenation
+            if (!(rhs instanceof EBinary) || !((EBinary)rhs).cat) { // check to see if the rhs has already done a concatenation
+                writer.writeAppendStrings(rhs.actual); // append the rhs's value since it's hasn't already
             }
 
             writer.writeToStrings(); // put the value for string concat onto the stack

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

@@ -21,7 +21,9 @@ package org.elasticsearch.painless;
 
 import static org.elasticsearch.painless.WriterConstants.MAX_INDY_STRING_CONCAT_ARGS;
 
+import java.util.HashMap;
 import java.util.Locale;
+import java.util.Map;
 
 public class StringTests extends ScriptTestCase {
 
@@ -222,6 +224,17 @@ public class StringTests extends ScriptTestCase {
         });
     }
 
+    public void testComplexCompoundAssignment() {
+        Map<String, Object> params = new HashMap<>();
+        Map<String, Object> ctx = new HashMap<>();
+        ctx.put("_id", "somerandomid");
+        params.put("ctx", ctx);
+
+        assertEquals("somerandomid.somerandomid", exec("ctx._id += '.' + ctx._id", params, false));
+        assertEquals("somerandomid.somerandomid", exec("String x = 'somerandomid'; x += '.' + x"));
+        assertEquals("somerandomid.somerandomid", exec("def x = 'somerandomid'; x += '.' + x"));
+    }
+
     public void testAppendStringIntoMap() {
         assertEquals("nullcat", exec("def a = new HashMap(); a.cat += 'cat'"));
     }