[K/N][IR] Generate missing return when using Nothing-returning safe call as lambda result or function expression body

^KT-42832

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

@@ -40,8 +40,14 @@ internal class ReturnsInsertionLowering(val context: Context) : FileLoweringPass
                             }
                         }
                         is IrBlockBody -> {
-                            if (declaration is IrConstructor || declaration.returnType == context.irBuiltIns.unitType)
+                            if (declaration is IrConstructor || declaration.returnType == context.irBuiltIns.unitType) {
                                 body.statements += irReturn(irGetObject(symbols.unit))
+                            } else if (declaration.returnType == context.irBuiltIns.nothingNType) {
+                                val typeOperatorCall = body.statements.singleOrNull() as? IrTypeOperatorCall
+                                if (typeOperatorCall?.operator == IrTypeOperator.IMPLICIT_COERCION_TO_UNIT) {
+                                    body.statements[0] = irReturn(typeOperatorCall.argument)
+                                }
+                            }
                         }
                     }
                 }

kotlin-native/backend.native/tests/build.gradle

@@ -530,6 +530,16 @@ task sum_3const(type: KonanLocalTest) {
     source = "codegen/function/sum_3const.kt"
 }
 
+standaloneTest("function_nothingN_returning_safe_call1") {
+    flags = ["-g", "-entry", "codegen.function.nothingN_returning_safe_call1.main"]
+    source = "codegen/function/nothingNReturningSafeCall1.kt"
+}
+
+standaloneTest("function_nothingN_returning_safe_call2") {
+    flags = ["-g", "-entry", "codegen.function.nothingN_returning_safe_call2.main"]
+    source = "codegen/function/nothingNReturningSafeCall2.kt"
+}
+
 task codegen_controlflow_for_loops(type: KonanLocalTest) {
     source = "codegen/controlflow/for_loops.kt"
     goldValue = "01234\n0123\n43210\n\n024\n02\n420\n\n036\n03\n630\n\n024\n02\n420\n\n"

kotlin-native/backend.native/tests/codegen/function/nothingNReturningSafeCall1.kt

@@ -0,0 +1,18 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package codegen.function.nothingN_returning_safe_call1
+
+import kotlin.test.*
+
+fun Any.nothing(): Nothing {
+    while (true) {}
+}
+
+fun foo(obj: Any?): Nothing? = obj?.nothing()
+
+fun main() {
+    foo(null)
+}

kotlin-native/backend.native/tests/codegen/function/nothingNReturningSafeCall2.kt

@@ -0,0 +1,23 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package codegen.function.nothingN_returning_safe_call2
+
+import kotlin.test.*
+
+fun Any.nothing(): Nothing {
+    while (true) {}
+}
+
+fun foo() {
+    val block: (Any?) -> Nothing? = {
+        it?.nothing()
+    }
+    block(null)
+}
+
+fun main() {
+    foo()
+}