fix binary plus operator handling of Empty and String

Author: Andrin Meier

Rubberduck.Parsing/Preprocessing/BinaryPlusExpression.cs

@@ -19,7 +19,7 @@ public override IValue Evaluate()
             {
                 return null;
             }
-            if (left.ValueType == ValueType.String)
+            if (left.ValueType == ValueType.String || right.ValueType == ValueType.String)
             {
                 return new StringValue(left.AsString+ right.AsString);
             }

Rubberduck.Parsing/Preprocessing/NameExpression.cs

@@ -16,7 +16,7 @@ public NameExpression(
         public override IValue Evaluate()
         {
             var identifier = _identifier.Evaluate().AsString;
-            // Special case, identifier that does not exist is VBAEmpty.
+            // Special case, identifier that does not exist is Empty.
             // Could add them to the symbol table, but since they are all constants
             // they never change anyway.
             if (!_symbolTable.HasSymbol(identifier))

RubberduckTests/Preprocessing/VBAPreprocessorVisitorTests.cs

@@ -94,13 +94,19 @@ public void TestPlusOperator()
 #Const c = True + False + True
 #Const d = Nothing
 #Const e = ""5"" + ""4""
+#Const f = Empty + Empty
+#Const g = Empty + ""a""
+#Const h = ""a"" + Empty
 ";
             var result = Preprocess(code);
             Assert.AreEqual(1544.242m, result.Item1.Get("a").AsDecimal);
             Assert.AreEqual(new DateTime(2298, 3, 7), result.Item1.Get("b").AsDate);
             Assert.AreEqual(-2m, result.Item1.Get("c").AsDecimal);
             Assert.AreEqual(null, result.Item1.Get("d"));
             Assert.AreEqual("54", result.Item1.Get("e").AsString);
+            Assert.AreEqual(0m, result.Item1.Get("f").AsDecimal);
+            Assert.AreEqual("a", result.Item1.Get("g").AsString);
+            Assert.AreEqual("a", result.Item1.Get("h").AsString);
         }
 
         [TestMethod]