SerializersModule doesn't prefer either contextual or subclass over plugin-generated serializers

Describe the bug I have many serializers, which are build at runtime and customize the plugin-generated serializers. SerializersModule does not select them. It instead always uses the plugin-generated serializers.

To Reproduce

package com.issue

import kotlinx.serialization.SerialName
import kotlinx.serialization.Serializable
import kotlinx.serialization.encodeToString
import kotlinx.serialization.json.*
import kotlinx.serialization.modules.SerializersModule
import kotlinx.serialization.modules.contextual
import kotlinx.serialization.modules.polymorphic
import kotlinx.serialization.modules.subclass
import kotlin.test.Test
import kotlin.test.assertEquals

class IssueTest {

    @Serializable
    abstract class Project {
        abstract val name: String
    }

    @Serializable
    @SerialName("owned")
    class OwnedProject(override val name: String, val owner: String) : Project()

    class CustomOwnedProjectSerializer(private val customField: String) :
        JsonTransformingSerializer<OwnedProject>(OwnedProject.serializer()) {
        override fun transformSerialize(element: JsonElement): JsonElement {
            require(element is JsonObject)
            return JsonObject(
                mapOf(
                    "customType" to JsonPrimitive("project.owned"),
                    "customField" to JsonPrimitive(customField),
                    *element.entries.map { Pair(it.key, it.value) }.toTypedArray()
                )
            )
        }
    }

    val module = SerializersModule {
        contextual(CustomOwnedProjectSerializer("unicorn"))
        polymorphic(Project::class) {
            subclass(CustomOwnedProjectSerializer("unicorn"))
        }
    }

    val format = Json { serializersModule = module }


    @Test
    fun test() {
        val data = OwnedProject("kotlinx.coroutines", "kotlin")
        val expectedJson =
            """{"customType":"project.owned","customField":"unicorn","name":"kotlinx.coroutines","owner":"kotlin"}"""
        assertEquals(expectedJson, format.encodeToString(CustomOwnedProjectSerializer("unicorn"), data)) // works
        assertEquals(expectedJson, format.encodeToString<Project>(data)) // does not work
        assertEquals(expectedJson, format.encodeToString(data)) // does not work
    }
}

Expected behavior I expect, that SerializersModule prefers registered serializers over plugin-generated serializers.

Environment

• Kotlin version: 1.4.21
• Library version: 1.0.1
• Kotlin platforms: multiplatform (JVM/JS/Native)]
• Gradle version: 6.8

[shanshin]

This is a bug in polymorphism using JsonTransformingSerializer child serializer: #1329. As a temporary workaround, you may try to create a surrogate class (see https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/serializers.md#composite-serializer-via-surrogate).

[benkuly]

@sandwwraith any idea?