Do not treat uninitialized value as a reason to retain boxing

See testData/simpleUnitializedMerge.kt
On exit from `if` we merge value in variable with slot 1 (x):
- from `if` body we get BoxedBasicValue
- from outer block we get UNITIALIZED_VALUE

So we just suppose `x` is unitialized after `if`
and there's no need to mark BoxedValue as unsafe to remove
because it's anyway can't be used after `if`

 #KT-6842 Fixed

compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/boxing/BoxingInterpreter.java

@@ -210,6 +210,10 @@ public class BoxingInterpreter extends OptimizationBasicInterpreter {
     @Override
     @NotNull
     public BasicValue merge(@NotNull BasicValue v, @NotNull BasicValue w) {
+        if (v == BasicValue.UNINITIALIZED_VALUE || w == BasicValue.UNINITIALIZED_VALUE) {
+            return BasicValue.UNINITIALIZED_VALUE;
+        }
+
         if (v instanceof BoxedBasicValue && ((BoxedBasicValue) v).typeEquals(w)) {
             onMergeSuccess((BoxedBasicValue) v, (BoxedBasicValue) w);
             return v;

compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/boxing/RedundantBoxingMethodTransformer.java

@@ -114,6 +114,7 @@ public class RedundantBoxingMethodTransformer extends MethodTransformer {
             if (CollectionsKt.any(usedValues, new Function1<BasicValue, Boolean>() {
                 @Override
                 public Boolean invoke(BasicValue input) {
+                    if (input == BasicValue.UNINITIALIZED_VALUE) return false;
                     return input == null ||
                            !(input instanceof BoxedBasicValue) ||
                            !((BoxedBasicValue) input).isSafeToRemove() ||

compiler/testData/codegen/boxWithStdlib/boxingOptimization/foldRange.kt

@@ -0,0 +1,9 @@
+import kotlin.test.assertEquals
+
+fun box(): String {
+    val result = (1..5).fold(0) { x, y -> x + y }
+
+    assertEquals(15, result)
+
+    return "OK"
+}

compiler/testData/codegen/boxWithStdlib/boxingOptimization/kt6842.kt

@@ -0,0 +1,7 @@
+import kotlin.test.assertEquals
+
+fun box(): String {
+    val x = (10L..50).map { it * 40L }
+    assertEquals(400L, x.first())
+    return "OK"
+}

compiler/testData/codegen/boxWithStdlib/boxingOptimization/simpleUninitializedMerge.kt

@@ -0,0 +1,12 @@
+import kotlin.test.assertEquals
+
+fun box(): String {
+    var result = 0
+    if (1 == 1) {
+        val x: Int? = 1
+        result += x!!
+    }
+
+    assertEquals(1, result)
+    return "OK"
+}

compiler/testData/codegen/bytecodeText/boxingOptimization/kt6842.kt

@@ -0,0 +1,17 @@
+fun box(): Long {
+    val x1 = (1..5).map { it * 40 }
+    val x2 = (1..5).fold(0) { x, y -> x + y }
+    val x3 = (1..5).reduce { x, y -> x + y }
+    val x4 = (1..5).count { it > 0 }
+
+    val y1 = (1L..5L).map { it * 40L }
+    val y2 = (1L..5L).fold(0L) { x, y -> x + y }
+    val y3 = (1L..5L).reduce { x, y -> x + y }
+    val y4 = (1L..5L).count { it > 0L }
+
+    return (x1.first() + x2 + x3 + x4).toLong() + y1.first() + y2 + y3 + y4.toLong()
+}
+
+// 5 nextLong
+// 5 nextInt
+// 0 next\s*\(

compiler/testData/codegen/bytecodeText/boxingOptimization/simpleUninitializedMerge.kt

@@ -0,0 +1,10 @@
+fun box(): String {
+    var result = 0
+    if (1 == 1) {
+        val x: Int? = 1
+        result += x!!
+    }
+    return "OK"
+}
+
+// 0 java/lang/Integer.valueOf

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

@@ -376,6 +376,12 @@ public class BytecodeTextTestGenerated extends AbstractBytecodeTextTest {
             doTest(fileName);
         }
 
+        @TestMetadata("kt6842.kt")
+        public void testKt6842() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/boxingOptimization/kt6842.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("kt7224.kt")
         public void testKt7224() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/boxingOptimization/kt7224.kt");
@@ -412,6 +418,12 @@ public class BytecodeTextTestGenerated extends AbstractBytecodeTextTest {
             doTest(fileName);
         }
 
+        @TestMetadata("simpleUninitializedMerge.kt")
+        public void testSimpleUninitializedMerge() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/boxingOptimization/simpleUninitializedMerge.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("unsafeRemoving.kt")
         public void testUnsafeRemoving() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/boxingOptimization/unsafeRemoving.kt");

compiler/tests/org/jetbrains/kotlin/codegen/generated/BlackBoxWithStdlibCodegenTestGenerated.java

@@ -337,6 +337,12 @@ public class BlackBoxWithStdlibCodegenTestGenerated extends AbstractBlackBoxCode
             doTestWithStdlib(fileName);
         }
 
+        @TestMetadata("foldRange.kt")
+        public void testFoldRange() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/boxingOptimization/foldRange.kt");
+            doTestWithStdlib(fileName);
+        }
+
         @TestMetadata("kt5493.kt")
         public void testKt5493() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/boxingOptimization/kt5493.kt");
@@ -361,6 +367,12 @@ public class BlackBoxWithStdlibCodegenTestGenerated extends AbstractBlackBoxCode
             doTestWithStdlib(fileName);
         }
 
+        @TestMetadata("kt6842.kt")
+        public void testKt6842() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/boxingOptimization/kt6842.kt");
+            doTestWithStdlib(fileName);
+        }
+
         @TestMetadata("nullCheck.kt")
         public void testNullCheck() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/boxingOptimization/nullCheck.kt");
@@ -385,6 +397,12 @@ public class BlackBoxWithStdlibCodegenTestGenerated extends AbstractBlackBoxCode
             doTestWithStdlib(fileName);
         }
 
+        @TestMetadata("simpleUninitializedMerge.kt")
+        public void testSimpleUninitializedMerge() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/boxingOptimization/simpleUninitializedMerge.kt");
+            doTestWithStdlib(fileName);
+        }
+
         @TestMetadata("unsafeRemoving.kt")
         public void testUnsafeRemoving() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/boxingOptimization/unsafeRemoving.kt");