[FIR2IR] Keep redundant cast on 'this' for local anonymous function

#KT-42517 Fixed

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrVisitor.kt

@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.fir.backend
 
 import org.jetbrains.kotlin.KtNodeTypes
 import org.jetbrains.kotlin.descriptors.ClassKind
+import org.jetbrains.kotlin.descriptors.DescriptorVisibilities
 import org.jetbrains.kotlin.descriptors.Modality
 import org.jetbrains.kotlin.descriptors.Visibilities
 import org.jetbrains.kotlin.fir.*
@@ -423,8 +424,10 @@ class Fir2IrVisitor(
         return visitQualifiedAccessExpression(thisReceiverExpression, data)
     }
 
-    private fun implicitCastOrExpression(original: IrExpression, castType: IrType): IrExpression {
-        if (original.type == castType) return original
+    private fun implicitCastOrExpression(original: IrExpression, castType: IrType, isThisInLocalFun: Boolean = false): IrExpression {
+        // If the original is a "this" in a local function and original.type is the same as castType,
+        // we still want to keep the cast. See kt-42517
+        if (original.type == castType && !isThisInLocalFun) return original
         return IrTypeOperatorCallImpl(
             original.startOffset,
             original.endOffset,
@@ -485,7 +488,9 @@ class Fir2IrVisitor(
                 }
             }
         }
-        return implicitCastOrExpression(value, castTypeRef.toIrType())
+        val isLocalFun = ((value as? IrGetValue)?.symbol?.owner?.parent as? IrFunction)?.visibility == DescriptorVisibilities.LOCAL
+        return implicitCastOrExpression(value, castTypeRef.toIrType(),
+                                        isLocalFun && expressionWithSmartcast.originalExpression is FirThisReceiverExpression)
     }
 
     override fun visitExpressionWithSmartcast(expressionWithSmartcast: FirExpressionWithSmartcast, data: Any?): IrElement {

compiler/fir/fir2ir/tests/org/jetbrains/kotlin/codegen/ir/FirBlackBoxCodegenTestGenerated.java

@@ -30049,6 +30049,11 @@ public class FirBlackBoxCodegenTestGenerated extends AbstractFirBlackBoxCodegenT
             runTest("compiler/testData/codegen/box/smartCasts/kt19100.kt");
         }
 
+        @TestMetadata("kt42517.kt")
+        public void testKt42517() throws Exception {
+            runTest("compiler/testData/codegen/box/smartCasts/kt42517.kt");
+        }
+
         @TestMetadata("lambdaArgumentWithoutType.kt")
         public void testLambdaArgumentWithoutType() throws Exception {
             runTest("compiler/testData/codegen/box/smartCasts/lambdaArgumentWithoutType.kt");

compiler/testData/codegen/box/smartCasts/kt42517.kt

@@ -0,0 +1,19 @@
+// WITH_RUNTIME
+// FULL_JDK
+// IGNORE_BACKEND: JS_IR
+// IGNORE_BACKEND: JS
+
+fun Any.copyValueIfNeeded(): Any {
+    return when (this) {
+        is Array<*> -> java.lang.reflect.Array.newInstance(this::class.java.componentType, size).apply {
+            this as Array<Any?>
+            (this@copyValueIfNeeded as Array<Any?>).forEachIndexed { i, value -> this[i] = value?.copyValueIfNeeded() }
+        }
+        else -> this
+    }
+}
+
+fun box(): String {
+    val res = arrayOf("FAIL", "OK").copyValueIfNeeded() as Array<String>
+    return res[1]
+}

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

@@ -31820,6 +31820,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             runTest("compiler/testData/codegen/box/smartCasts/kt19100.kt");
         }
 
+        @TestMetadata("kt42517.kt")
+        public void testKt42517() throws Exception {
+            runTest("compiler/testData/codegen/box/smartCasts/kt42517.kt");
+        }
+
         @TestMetadata("lambdaArgumentWithoutType.kt")
         public void testLambdaArgumentWithoutType() throws Exception {
             runTest("compiler/testData/codegen/box/smartCasts/lambdaArgumentWithoutType.kt");

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

@@ -29454,6 +29454,11 @@ public class LightAnalysisModeTestGenerated extends AbstractLightAnalysisModeTes
             runTest("compiler/testData/codegen/box/smartCasts/kt19100.kt");
         }
 
+        @TestMetadata("kt42517.kt")
+        public void testKt42517() throws Exception {
+            runTest("compiler/testData/codegen/box/smartCasts/kt42517.kt");
+        }
+
         @TestMetadata("lambdaArgumentWithoutType.kt")
         public void testLambdaArgumentWithoutType() throws Exception {
             runTest("compiler/testData/codegen/box/smartCasts/lambdaArgumentWithoutType.kt");

compiler/tests/org/jetbrains/kotlin/codegen/ir/IrBlackBoxCodegenTestGenerated.java

@@ -30049,6 +30049,11 @@ public class IrBlackBoxCodegenTestGenerated extends AbstractIrBlackBoxCodegenTes
             runTest("compiler/testData/codegen/box/smartCasts/kt19100.kt");
         }
 
+        @TestMetadata("kt42517.kt")
+        public void testKt42517() throws Exception {
+            runTest("compiler/testData/codegen/box/smartCasts/kt42517.kt");
+        }
+
         @TestMetadata("lambdaArgumentWithoutType.kt")
         public void testLambdaArgumentWithoutType() throws Exception {
             runTest("compiler/testData/codegen/box/smartCasts/lambdaArgumentWithoutType.kt");

js/js.tests/test/org/jetbrains/kotlin/js/test/es6/semantics/IrJsCodegenBoxES6TestGenerated.java

@@ -24340,6 +24340,11 @@ public class IrJsCodegenBoxES6TestGenerated extends AbstractIrJsCodegenBoxES6Tes
             runTest("compiler/testData/codegen/box/smartCasts/kt19100.kt");
         }
 
+        @TestMetadata("kt42517.kt")
+        public void testKt42517() throws Exception {
+            runTest("compiler/testData/codegen/box/smartCasts/kt42517.kt");
+        }
+
         @TestMetadata("lambdaArgumentWithoutType.kt")
         public void testLambdaArgumentWithoutType() throws Exception {
             runTest("compiler/testData/codegen/box/smartCasts/lambdaArgumentWithoutType.kt");

js/js.tests/test/org/jetbrains/kotlin/js/test/ir/semantics/IrJsCodegenBoxTestGenerated.java

@@ -24340,6 +24340,11 @@ public class IrJsCodegenBoxTestGenerated extends AbstractIrJsCodegenBoxTest {
             runTest("compiler/testData/codegen/box/smartCasts/kt19100.kt");
         }
 
+        @TestMetadata("kt42517.kt")
+        public void testKt42517() throws Exception {
+            runTest("compiler/testData/codegen/box/smartCasts/kt42517.kt");
+        }
+
         @TestMetadata("lambdaArgumentWithoutType.kt")
         public void testLambdaArgumentWithoutType() throws Exception {
             runTest("compiler/testData/codegen/box/smartCasts/lambdaArgumentWithoutType.kt");

js/js.tests/test/org/jetbrains/kotlin/js/test/semantics/JsCodegenBoxTestGenerated.java

@@ -24355,6 +24355,11 @@ public class JsCodegenBoxTestGenerated extends AbstractJsCodegenBoxTest {
             runTest("compiler/testData/codegen/box/smartCasts/kt19100.kt");
         }
 
+        @TestMetadata("kt42517.kt")
+        public void testKt42517() throws Exception {
+            runTest("compiler/testData/codegen/box/smartCasts/kt42517.kt");
+        }
+
         @TestMetadata("lambdaArgumentWithoutType.kt")
         public void testLambdaArgumentWithoutType() throws Exception {
             runTest("compiler/testData/codegen/box/smartCasts/lambdaArgumentWithoutType.kt");