Fix for KT-4301: Too many StringBuilders on string concatenations

#KT-4301 Fixed

compiler/backend/src/org/jetbrains/jet/codegen/ExpressionCodegen.java

@@ -3043,9 +3043,8 @@ public class ExpressionCodegen extends JetVisitor<StackValue, StackValue> implem
                 JetExpression left = binaryExpression.getLeft();
                 JetExpression right = binaryExpression.getRight();
                 Type leftType = expressionType(left);
-                Type rightType = expressionType(right);
 
-                if (leftType.equals(JAVA_STRING_TYPE) && rightType.equals(JAVA_STRING_TYPE)) {
+                if (leftType.equals(JAVA_STRING_TYPE)) {
                     invokeAppend(left);
                     invokeAppend(right);
                     return;

compiler/testData/codegen/box/strings/stringBuilderAppend.kt

@@ -0,0 +1,16 @@
+class A() {
+
+    override fun toString(): String {
+        return "A"
+    }
+}
+
+fun box() : String {
+
+    val s = "1" + "2" + 3 + 4L + 5.0 + 6F + '7' + A()
+
+    if (s != "12345.06.07A") return "fail $s"
+
+    return "OK"
+}
+

compiler/testData/codegen/bytecodeText/stringBuilderAppend.kt

@@ -0,0 +1,16 @@
+class A() {
+
+    override fun toString(): String {
+        return "A"
+    }
+}
+
+
+fun box() : String {
+
+    val s = "1" + "2" + 3 + 4L + 5.0 + 6F + '7' + A()
+
+    return s
+}
+
+// 1 NEW java/lang/StringBuilder

compiler/tests/org/jetbrains/jet/codegen/BytecodeTextTestGenerated.java

@@ -177,6 +177,12 @@ public class BytecodeTextTestGenerated extends AbstractBytecodeTextTest {
         doTest(fileName);
     }
     
+    @TestMetadata("stringBuilderAppend.kt")
+    public void testStringBuilderAppend() throws Exception {
+        String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/stringBuilderAppend.kt");
+        doTest(fileName);
+    }
+    
     @TestMetadata("topLevelFunWithDefaultArgs.kt")
     public void testTopLevelFunWithDefaultArgs() throws Exception {
         String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/topLevelFunWithDefaultArgs.kt");

compiler/tests/org/jetbrains/jet/codegen/generated/BlackBoxCodegenTestGenerated.java

@@ -5990,6 +5990,12 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             doTest(fileName);
         }
         
+        @TestMetadata("stringBuilderAppend.kt")
+        public void testStringBuilderAppend() throws Exception {
+            String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/box/strings/stringBuilderAppend.kt");
+            doTest(fileName);
+        }
+        
     }
     
     @TestMetadata("compiler/testData/codegen/box/super")