for completely empty if statement, put on stack the Tuple0 instance, not its condition (KT-2062)

#KT-2062 Fixed

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

@@ -245,7 +245,10 @@ public class ExpressionCodegen extends JetVisitor<StackValue, StackValue> {
 
         if (isEmptyExpression(thenExpression)) {
             if (isEmptyExpression(elseExpression)) {
-                condition.put(asmType, v);
+                if (!asmType.equals(JetTypeMapper.TUPLE0_TYPE)) {
+                    throw new CompilationException("Completely empty 'if' is expected to have Unit type", null, expression);
+                }
+                StackValue.putTuple0Instance(v);
                 return StackValue.onStack(asmType);
             }
             return generateSingleBranchIf(condition, elseExpression, false);

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

@@ -260,7 +260,7 @@ public abstract class StackValue {
         }
     }
 
-    protected static void putTuple0Instance(InstructionAdapter v) {
+    public static void putTuple0Instance(InstructionAdapter v) {
         v.visitFieldInsn(Opcodes.GETSTATIC, "jet/Tuple0", "INSTANCE", "Ljet/Tuple0;");
     }
 

compiler/testData/codegen/regressions/kt2062.kt

@@ -0,0 +1,5 @@
+fun box(): String {
+  val a = if(true) {
+  }
+  return if (a.toString() == "()") "OK" else "fail"
+}

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

@@ -346,4 +346,9 @@ public class ControlStructuresTest extends CodegenTestCase {
         createEnvironmentWithMockJdkAndIdeaAnnotations(CompilerSpecialMode.JDK_HEADERS);
         blackBoxFile("regressions/kt1742.kt");
     }
+
+    public void testKt2062() throws Exception {
+        createEnvironmentWithMockJdkAndIdeaAnnotations(CompilerSpecialMode.JDK_HEADERS);
+        blackBoxFile("regressions/kt2062.kt");
+    }
 }