jvm/1.8.20 returns KSerializer<List<String>> instead of KSerializer<List<String?>>

[simonegiacomelli]

Describe the bug
The serializer returned should be KSerializer<List> but the expected type is KSerializer<List<String?>>.
In other words, it doesn't honor the nullability of the first generic type of the list.
It happens only on jvm/1.8.20.
Workaround attached.

To Reproduce
This repository reproduces the issue.
Btw all the code needed is this:

import kotlinx.serialization.KSerializer
import kotlinx.serialization.descriptors.elementDescriptors
import kotlinx.serialization.serializer
import kotlin.reflect.typeOf
import kotlin.test.Test
import kotlin.test.assertTrue

class IssueTest {

    /** This fails with jvm/1.8.20 */
    @Test
    fun test_issue() {
        assertTrue(listOfNullable<String>().descriptor.elementDescriptors.first().isNullable)
    }

    /** This works with all the combinations mentioned, i.e., js/jvm/1.8.20 and js/jvm/1.8.10 */
    @Test
    fun test_workaround() {
        assertTrue(workaround<String>().descriptor.elementDescriptors.first().isNullable)
    }
}

inline fun <reified T> listOfNullable(): KSerializer<List<Any?>> = serializer<List<T?>>() as KSerializer<List<Any?>>
inline fun <reified T> workaround(): KSerializer<List<Any?>> = serializer(typeOf<List<T?>>()) as KSerializer<List<Any?>>

Expected behavior
It should return a KSerializer<List<String?>>

Environment

• Kotlin version: 1.8.20
• Library version: org.jetbrains.kotlinx:kotlinx-serialization-json:1.5.0
• Kotlin platforms: 1.8.20/jvm affected
• Gradle version: 7.4.2
• jvm: 1.8.0_312 (Amazon.com Inc. 25.312-b07)

[sandwwraith]

Fixed in Kotlin 1.8.21