KT-881 overoptimized + in string templates

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

@@ -1894,9 +1894,16 @@ public class ExpressionCodegen extends JetVisitor<StackValue, StackValue> {
         if (expr instanceof JetBinaryExpression) {
             final JetBinaryExpression binaryExpression = (JetBinaryExpression) expr;
             if (binaryExpression.getOperationToken() == JetTokens.PLUS) {
-                invokeAppend(binaryExpression.getLeft());
-                invokeAppend(binaryExpression.getRight());
-                return;
+                JetExpression left = binaryExpression.getLeft();
+                JetExpression right = binaryExpression.getRight();
+                Type leftType = expressionType(left);
+                Type rightType = expressionType(right);
+
+                if(leftType.equals(JetTypeMapper.JL_STRING_TYPE) && rightType.equals(JL_STRING_TYPE)) {
+                    invokeAppend(left);
+                    invokeAppend(right);
+                    return;
+                }
             }
         }
         Type exprType = expressionType(expr);

compiler/testData/codegen/regressions/kt881.jet

@@ -0,0 +1,6 @@
+fun box() : String {
+  val b = 1+1
+  if ("$b" != "2") return "fail"
+  if ("${1+1}" != "2") return "fail"
+  return "OK"
+}

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

@@ -9,4 +9,8 @@ public class StringsTest extends CodegenTestCase {
     public void testRawStrings() throws Exception {
         blackBoxFile("rawStrings.jet");
     }
+
+    public void testKt881() throws Exception {
+        blackBoxFile("regressions/kt881.jet");
+    }
 }