[K/N] Better handling of nested returns in RedundantCoercionsCleaner.kt

^KT-49356

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/lower/RedundantCoercionsCleaner.kt

@@ -11,6 +11,7 @@ import org.jetbrains.kotlin.backend.konan.DECLARATION_ORIGIN_INLINE_CLASS_SPECIA
 import org.jetbrains.kotlin.backend.konan.getInlinedClassNative
 import org.jetbrains.kotlin.backend.konan.ir.isBoxOrUnboxCall
 import org.jetbrains.kotlin.ir.IrElement
+import org.jetbrains.kotlin.ir.IrStatement
 import org.jetbrains.kotlin.ir.declarations.IrClass
 import org.jetbrains.kotlin.ir.declarations.IrFile
 import org.jetbrains.kotlin.ir.declarations.IrFunction
@@ -161,6 +162,16 @@ internal class RedundantCoercionsCleaner(val context: Context) : FileLoweringPas
                                         statements = statements,
                                         inlineFunctionSymbol = inlineFunctionSymbol)
                             }
+                            /*
+                             * Visitor below requires being very careful when changing it.
+                             * It heavily relies on implementation of fold and getFullExpression functions,
+                             * in particular it uses fact, that parts of foldedReturnableBlockValues will be replaced
+                             * simultaneously with IR.
+                             *
+                             * For example, this requires replacing ReturnableBlock symbols in separate pass,
+                             * because it needs new return nodes to be created, which breaks changing it's content in
+                             * foldedReturnableBlockValues.
+                             */
                             transformedReturnableBlock.transformChildrenVoid(object: IrElementTransformerVoid() {
                                 override fun visitExpression(expression: IrExpression): IrExpression {
                                     expression.transformChildrenVoid()
@@ -169,7 +180,8 @@ internal class RedundantCoercionsCleaner(val context: Context) : FileLoweringPas
                                     }
                                     return expression
                                 }
-
+                            })
+                            transformedReturnableBlock.transformChildrenVoid(object: IrElementTransformerVoid() {
                                 override fun visitReturn(expression: IrReturn): IrExpression {
                                     expression.transformChildrenVoid(this)
                                     return if (expression.returnTargetSymbol != oldSymbol)

kotlin-native/backend.native/tests/codegen/inline/redundantCoercionsCleaner.kt

@@ -33,3 +33,17 @@ inline fun foo2(): Int {
 @Test fun runTest2() {
     assertEquals(2, bar2())
 }
+
+// Test for https://youtrack.jetbrains.com/issue/KT-49356.
+
+inline fun foo3(): Int {
+    return (return 3)
+}
+
+fun bar3(): Any {
+    return foo3()
+}
+
+@Test fun runTest3() {
+    assertEquals(3, bar3())
+}