[JS IR BE] Fix Throwable ancestor transformation

Make sure that `message` and `cause` are properly configured

 - synchronize IR BE and Legacy behaviour
 - fix corresponding IR lowering
 - fix JS IR core runtime
 - add test
 - fix KT-39964

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/ThrowableLowering.kt

@@ -24,12 +24,14 @@ class ThrowableLowering(
     val context: JsIrBackendContext,
     val extendThrowableFunction: IrSimpleFunctionSymbol
 ) : BodyLoweringPass {
-    private val nothingNType get() = context.irBuiltIns.nothingNType
+    private val nothingNType = context.irBuiltIns.nothingNType
 
     private val throwableConstructors = context.throwableConstructors
     private val newThrowableFunction = context.newThrowableSymbol
+    private val jsUndefined = context.intrinsics.jsUndefined.symbol
 
     fun nullValue(): IrExpression = IrConstImpl.constNull(UNDEFINED_OFFSET, UNDEFINED_OFFSET, nothingNType)
+    fun undefinedValue(): IrExpression = IrCallImpl(UNDEFINED_OFFSET, UNDEFINED_OFFSET, nothingNType, jsUndefined, 0, 0)
 
     data class ThrowableArguments(
         val message: IrExpression,
@@ -44,7 +46,7 @@ class ThrowableLowering(
 
     private fun IrFunctionAccessExpression.extractThrowableArguments(): ThrowableArguments =
         when (valueArgumentsCount) {
-            0 -> ThrowableArguments(nullValue(), nullValue())
+            0 -> ThrowableArguments(nullValue(), undefinedValue())
             2 -> ThrowableArguments(
                 message = getValueArgument(0)!!,
                 cause = getValueArgument(1)!!
@@ -53,10 +55,10 @@ class ThrowableLowering(
                 val arg = getValueArgument(0)!!
                 val parameter = symbol.owner.valueParameters[0]
                 when {
-                    parameter.type.isNullableString() -> ThrowableArguments(message = arg, cause = nullValue())
+                    parameter.type.isNullableString() -> ThrowableArguments(message = arg, cause = undefinedValue())
                     else -> {
                         assert(parameter.type.makeNotNull().isThrowable())
-                        ThrowableArguments(message = nullValue(), cause = arg)
+                        ThrowableArguments(message = undefinedValue(), cause = arg)
                     }
                 }
             }

js/js.tests/test/org/jetbrains/kotlin/js/test/es6/semantics/IrBoxJsES6TestGenerated.java

@@ -7675,6 +7675,11 @@ public class IrBoxJsES6TestGenerated extends AbstractIrBoxJsES6Test {
         public void testStringPlus() throws Exception {
             runTest("js/js.translator/testData/box/standardClasses/stringPlus.kt");
         }
+
+        @TestMetadata("throwableCtor.kt")
+        public void testThrowableCtor() throws Exception {
+            runTest("js/js.translator/testData/box/standardClasses/throwableCtor.kt");
+        }
     }
 
     @TestMetadata("js/js.translator/testData/box/superCall")

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

@@ -7675,6 +7675,11 @@ public class IrBoxJsTestGenerated extends AbstractIrBoxJsTest {
         public void testStringPlus() throws Exception {
             runTest("js/js.translator/testData/box/standardClasses/stringPlus.kt");
         }
+
+        @TestMetadata("throwableCtor.kt")
+        public void testThrowableCtor() throws Exception {
+            runTest("js/js.translator/testData/box/standardClasses/throwableCtor.kt");
+        }
     }
 
     @TestMetadata("js/js.translator/testData/box/superCall")

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

@@ -7705,6 +7705,11 @@ public class BoxJsTestGenerated extends AbstractBoxJsTest {
         public void testStringPlus() throws Exception {
             runTest("js/js.translator/testData/box/standardClasses/stringPlus.kt");
         }
+
+        @TestMetadata("throwableCtor.kt")
+        public void testThrowableCtor() throws Exception {
+            runTest("js/js.translator/testData/box/standardClasses/throwableCtor.kt");
+        }
     }
 
     @TestMetadata("js/js.translator/testData/box/superCall")

js/js.translator/testData/box/standardClasses/throwableCtor.kt

@@ -0,0 +1,10 @@
+// EXPECTED_REACHABLE_NODES: 1237
+
+// KT-39964
+
+
+fun box(): String {
+    val e = Throwable(null, IllegalStateException("fail"))
+    if (e.message != null) return "FAIL"
+    return "OK"
+}

libraries/stdlib/js-ir/runtime/coreRuntime.kt

@@ -89,7 +89,7 @@ internal fun captureStack(instance: Throwable, constructorFunction: Any) {
 
 internal fun newThrowable(message: String?, cause: Throwable?): Throwable {
     val throwable = js("new Error()")
-    throwable.message = message ?: cause?.toString() ?: undefined
+    throwable.message = if (isUndefined(message)) cause?.toString() else message
     throwable.cause = cause
     throwable.name = "Throwable"
     return throwable.unsafeCast<Throwable>()
@@ -102,7 +102,17 @@ internal fun extendThrowable(this_: dynamic, message: String?, cause: Throwable?
 
 internal fun setPropertiesToThrowableInstance(this_: dynamic, message: String?, cause: Throwable?) {
     if (!hasOwnPrototypeProperty(this_, "message")) {
-        this_.message = message ?: cause?.toString() ?: undefined
+        @Suppress("IfThenToElvis")
+        this_.message = if (message == null) {
+            @Suppress("SENSELESS_COMPARISON")
+            if (message !== null) {
+                // undefined
+                cause?.toString() ?: undefined
+            } else {
+                // real null
+                message
+            }
+        } else message
     }
     if (!hasOwnPrototypeProperty(this_, "cause")) {
         this_.cause = cause
@@ -122,5 +132,8 @@ internal fun errorCode(description: String): Nothing {
     throw IllegalStateException(description)
 }
 
+@Suppress("SENSELESS_COMPARISON")
+internal fun isUndefined(value: dynamic): Boolean = value === undefined
+
 internal fun <T, R> boxIntrinsic(@Suppress("UNUSED_PARAMETER") x: T): R = error("Should be lowered")
 internal fun <T, R> unboxIntrinsic(@Suppress("UNUSED_PARAMETER") x: T): R = error("Should be lowered")