Primitive values cannot be serialized to JsonElement

[oxc]

Describe the bug
While Json.plain.stringify can be used on primitives and complex objects alike, Json.plain.toJson fails on primitive objects with an exception:

kotlinx.serialization.SerializationException: No tag in stack for requested element
	at kotlinx.serialization.TaggedEncoder.popTag(Tagged.kt:160)
	at kotlinx.serialization.TaggedEncoder.encodeBoolean(Tagged.kt:95)
	at kotlinx.serialization.internal.BooleanSerializer.serialize(Primitives.kt:54)
	at kotlinx.serialization.internal.BooleanSerializer.serialize(Primitives.kt:51)
	at kotlinx.serialization.json.internal.AbstractJsonTreeOutput.encodeSerializableValue(TreeJsonOutput.kt:174)
	at kotlinx.serialization.CoreKt.encode(Core.kt:73)
	at kotlinx.serialization.json.internal.TreeJsonOutputKt.writeJson(TreeJsonOutput.kt:29)
	at kotlinx.serialization.json.Json.toJson(Json.kt:129)

To Reproduce
Run (e.g. ./gradlew run) the attached demo project from
primitive-serialization.zip

Expected behavior
I would expect both Strings and Booleans to be serialized to JsonPrimitive instances.

Environment

• Kotlin version: 1.3.31
• Library version: 0.11.0
• Kotlin platforms: JVM
• Gradle version: 4.10.3

[yshrsmz]

Kotlin: 1.3.31
Serialization: 0.11.0
Platform: JVM
Gradle: 5.1.1

I'm having similar issue.
In my case, Json.nonstrict.toJson with NullableSerializer or EnumSerializer fails with similar exception.

for NullableSerializer:

@Serializable
data class UserProfile(val id:String)

fun test() {
  val jsonElement = Json.nonstrict.toJson<UserProfile?>(NullableSerializer(UserProfile.serializer()), null)
  println("jsonElement: $jsonElement")
}

 Caused by: kotlinx.serialization.SerializationException: No tag in stack for requested element
        at kotlinx.serialization.TaggedEncoder.popTag(Tagged.kt:160)
        at kotlinx.serialization.TaggedEncoder.encodeNull(Tagged.kt:92)
        at kotlinx.serialization.internal.NullableSerializer.serialize(NullableSerializer.kt:39)
        at kotlinx.serialization.json.internal.AbstractJsonTreeOutput.encodeSerializableValue(TreeJsonOutput.kt:174)
        at kotlinx.serialization.CoreKt.encode(Core.kt:73)
        at kotlinx.serialization.json.internal.TreeJsonOutputKt.writeJson(TreeJsonOutput.kt:29)
        at kotlinx.serialization.json.Json.toJson(Json.kt:129)

for EnumSerializer

enum class CollaboratorAffiliation {
    OUTSIDE, DIRECT, ALL
}

fun test() {
  val el = Json.nonstrict.toJson(EnumSerializer(CollaboratorAffiliation::class), CollaboratorAffiliation.ALL)
  println("el: $el")
}

Caused by: kotlinx.serialization.SerializationException: No tag in stack for requested element
        at kotlinx.serialization.TaggedEncoder.popTag(Tagged.kt:160)
        at kotlinx.serialization.TaggedEncoder.encodeEnum(Tagged.kt:108)
        at kotlinx.serialization.internal.CommonEnumSerializer.serialize(Enums.kt:60)
        at kotlinx.serialization.json.internal.AbstractJsonTreeOutput.encodeSerializableValue(TreeJsonOutput.kt:174)
        at kotlinx.serialization.CoreKt.encode(Core.kt:73)
        at kotlinx.serialization.json.internal.TreeJsonOutputKt.writeJson(TreeJsonOutput.kt:29)
        at kotlinx.serialization.json.Json.toJson(Json.kt:129)

[oxc]

Ok, so this seems to apply to all cases where the value is not serialized to a json object (or array, probably), including null.