Mantis#1469. Thank you kindly, Mikem for a patch that addresses:

Currently LSL code such as below does not compile on OpenSim, but compiles fine in Second Life:
list mylist = [];
mylist += [1, 2, 3];
mylist += "four";
list newlist = mylist + 5.0;
The problem is that the LSL_Types.list class does not have an operator for adding a string to a list.
I am including a patch which implements adding a string, integer or float to a list.
I am also including tests. The file LSL_TypesTestList.cs belongs in 
OpenSim/Tests/OpenSim/Region/ScriptEngine/Common/.

OpenSim/Region/ScriptEngine/Common/LSL_Types.cs

@@ -425,6 +425,30 @@ namespace OpenSim.Region.ScriptEngine.Common
                 return new list(tmp);
             }
 
+            private void ExtendAndAdd(object o)
+            {
+                Array.Resize(ref m_data, Length + 1);
+                m_data.SetValue(o, Length - 1);
+            }
+
+            public static list operator +(list a, string s)
+            {
+                a.ExtendAndAdd(s);
+                return a;
+            }
+
+            public static list operator +(list a, int i)
+            {
+                a.ExtendAndAdd(i);
+                return a;
+            }
+
+            public static list operator +(list a, double d)
+            {
+                a.ExtendAndAdd(d);
+                return a;
+            }
+
             public void Add(object o)
             {
                 object[] tmp;

OpenSim/Tests/OpenSim/Region/ScriptEngine/Common/LSL_TypesTestList.cs

@@ -0,0 +1,101 @@
+/*
+ * Copyright (c) Contributors, http://opensimulator.org/
+ * See CONTRIBUTORS.TXT for a full list of copyright holders.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of the OpenSim Project nor the
+ *       names of its contributors may be used to endorse or promote products
+ *       derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE CONTRIBUTORS BE LIABLE FOR ANY
+ * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+using System.Collections.Generic;
+using NUnit.Framework;
+using OpenSim.Tests.Common;
+using OpenSim.Region.ScriptEngine.Common;
+
+namespace OpenSim.Region.ScriptEngine.Common.Tests
+{
+    /// <summary>
+    /// Tests the LSL_Types.list class.
+    /// </summary>
+    [TestFixture]
+    public class LSL_TypesTestList
+    {
+        /// <summary>
+        /// Tests concatenating a string to a list.
+        /// </summary>
+        [Test]
+        public void TestConcatenateString()
+        {
+            LSL_Types.list testList = new LSL_Types.list(1, 'a', "test");
+            testList += "addition";
+
+            Assert.AreEqual(4, testList.Length);
+            Assert.AreEqual("addition", testList.Data[3]);
+            Assert.AreEqual(typeof(System.String), testList.Data[3].GetType());
+
+            LSL_Types.list secondTestList = testList + "more";
+
+            Assert.AreEqual(5, secondTestList.Length);
+            Assert.AreEqual("more", secondTestList.Data[4]);
+            Assert.AreEqual(typeof(System.String), secondTestList.Data[4].GetType());
+        }
+
+        /// <summary>
+        /// Tests concatenating an integer to a list.
+        /// </summary>
+        [Test]
+        public void TestConcatenateInteger()
+        {
+            LSL_Types.list testList = new LSL_Types.list(1, 'a', "test");
+            testList += 20;
+
+            Assert.AreEqual(4, testList.Length);
+            Assert.AreEqual(20, testList.Data[3]);
+            Assert.AreEqual(typeof(int), testList.Data[3].GetType());
+
+            LSL_Types.list secondTestList = testList + 2;
+
+            Assert.AreEqual(5, secondTestList.Length);
+            Assert.AreEqual(2, secondTestList.Data[4]);
+            Assert.AreEqual(typeof(int), secondTestList.Data[4].GetType());
+        }
+
+        /// <summary>
+        /// Tests concatenating an integer to a list.
+        /// </summary>
+        [Test]
+        public void TestConcatenateDouble()
+        {
+            LSL_Types.list testList = new LSL_Types.list(1, 'a', "test");
+            testList += 2.0;
+
+            Assert.AreEqual(4, testList.Length);
+            Assert.AreEqual(2.0, testList.Data[3]);
+            Assert.AreEqual(typeof(double), testList.Data[3].GetType());
+
+            LSL_Types.list secondTestList = testList + 0.04;
+
+            Assert.AreEqual(5, secondTestList.Length);
+            Assert.AreEqual(0.04, secondTestList.Data[4]);
+            Assert.AreEqual(typeof(double), secondTestList.Data[4].GetType());
+        }
+    }
+}