Correctly support nullability in type arguments for serializer<T>() intrinsic.

Nullability info should be added to TYPE_OF operation marker.

Fixes https://github.com/Kotlin/kotlinx.serialization/issues/2265

plugins/kotlinx-serialization/testData/boxIr/intrinsicsNullable.kt

@@ -0,0 +1,49 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+import kotlinx.serialization.internal.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.modules.*
+import java.lang.AssertionError
+
+inline fun <reified T: Any?> listOfNullable(): KSerializer<List<T?>> = serializer<List<T?>>()
+inline fun <reified T: Any> listOfNullableWithNonNullBound(): KSerializer<List<T?>> = serializer<List<T?>>()
+inline fun <reified T> listOfNullableNoExplicitBound(): KSerializer<List<T?>> = serializer<List<T?>>()
+inline fun <reified T> listOfNullableWithCast(): KSerializer<List<Any?>> = serializer<List<T?>>() as KSerializer<List<Any?>>
+inline fun <reified T> listOfUnspecifiedNullability(): KSerializer<List<T>> = serializer<List<T>>()
+
+inline fun <reified T> getSer(module: SerializersModule): KSerializer<T> {
+    return module.serializer<T>()
+}
+
+fun check(shouldBeNullable: Boolean, descriptor: SerialDescriptor) {
+    if (shouldBeNullable == descriptor.isNullable) return
+    if (shouldBeNullable) throw java.lang.AssertionError("Should be nullable, but is not: $descriptor")
+    throw java.lang.AssertionError("Should not be nullable, but it is: $descriptor")
+}
+
+fun box(): String {
+    check(false, serializer<String>().descriptor)
+    check(true, serializer<String?>().descriptor)
+
+    check(false, serializer<List<String>>().descriptor.elementDescriptors.first())
+    check(true, serializer<List<String?>>().descriptor.elementDescriptors.first())
+    check(true, serializer<List<String>?>().descriptor)
+
+    check(true, listOfNullable<String>().descriptor.elementDescriptors.first())
+    check(true, listOfNullableNoExplicitBound<String>().descriptor.elementDescriptors.first())
+    check(true, listOfNullableWithNonNullBound<String>().descriptor.elementDescriptors.first())
+    check(true, listOfNullableWithCast<String>().descriptor.elementDescriptors.first())
+
+    check(false, listOfUnspecifiedNullability<String>().descriptor.elementDescriptors.first())
+    check(true, listOfUnspecifiedNullability<String?>().descriptor.elementDescriptors.first())
+
+    val module = EmptySerializersModule()
+    check(false, getSer<String>(module).descriptor)
+    check(true, getSer<String?>(module).descriptor)
+
+    return "OK"
+}

plugins/kotlinx-serialization/testData/boxIr/intrinsicsStarProjections.kt

@@ -47,4 +47,4 @@ fun box(): String {
         getListSer<Box<*>>()
     }
     return "OK"
-}
+}