[FIR2IR] Fix generation of val initialization after smart-cast

If neither setter nor backing field were found for an assignment call,
search overridden properties for backing fields.

^KT-57105 Fixed

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/generators/CallAndReferenceGenerator.kt

@@ -569,7 +569,14 @@ class CallAndReferenceGenerator(
                     is IrPropertySymbol -> {
                         val irProperty = symbol.owner
                         val setter = irProperty.setter
-                        val backingField = irProperty.backingField
+                        var backingField = irProperty.backingField
+
+                        // If we found neither a setter nor a backing field, check if we have an override (possibly fake) of a val with
+                        // backing field. This can happen in a class initializer where `this` was smart-casted. See KT-57105.
+                        if (setter == null && backingField == null) {
+                            backingField = irProperty.overriddenSymbols.firstNotNullOfOrNull { it.owner.backingField }
+                        }
+
                         when {
                             setter != null -> IrCallImpl(
                                 startOffset, endOffset, type, setter.symbol,

compiler/fir/fir2ir/tests-gen/org/jetbrains/kotlin/test/runners/codegen/FirLightTreeBlackBoxCodegenTestGenerated.java

@@ -48995,6 +48995,12 @@ public class FirLightTreeBlackBoxCodegenTestGenerated extends AbstractFirLightTr
             runTest("compiler/testData/codegen/box/smartCasts/nullSmartCast.kt");
         }
 
+        @Test
+        @TestMetadata("propertyInitializationAfterSmartCast.kt")
+        public void testPropertyInitializationAfterSmartCast() throws Exception {
+            runTest("compiler/testData/codegen/box/smartCasts/propertyInitializationAfterSmartCast.kt");
+        }
+
         @Test
         @TestMetadata("resolveToMemberOfSuperclass_overrideWithDifferentType.kt")
         public void testResolveToMemberOfSuperclass_overrideWithDifferentType() throws Exception {

compiler/fir/fir2ir/tests-gen/org/jetbrains/kotlin/test/runners/codegen/FirPsiBlackBoxCodegenTestGenerated.java

@@ -48995,6 +48995,12 @@ public class FirPsiBlackBoxCodegenTestGenerated extends AbstractFirPsiBlackBoxCo
             runTest("compiler/testData/codegen/box/smartCasts/nullSmartCast.kt");
         }
 
+        @Test
+        @TestMetadata("propertyInitializationAfterSmartCast.kt")
+        public void testPropertyInitializationAfterSmartCast() throws Exception {
+            runTest("compiler/testData/codegen/box/smartCasts/propertyInitializationAfterSmartCast.kt");
+        }
+
         @Test
         @TestMetadata("resolveToMemberOfSuperclass_overrideWithDifferentType.kt")
         public void testResolveToMemberOfSuperclass_overrideWithDifferentType() throws Exception {