Spring KotlinSerializer can not serialize ResolvableType.EmptyType.INSTANCE

Affects: Spring 6.0.0-M6

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Spring version: 6.0.0-M6

https://github.com/spring-projects/spring-framework/blob/ba0a835880581244f39e299b624c8a200cfbdf8a/spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/AbstractMessageConverterMethodProcessor.java#L189

In the above line, if the targetType is resolved as ResolvableType.EmptyType.INSTANCE, kotilnx serializer fails to serialize it since KotlinSerializationJsonHttpMessageConverter tries to find serializer of this instance of Type by using SerializerKt.serializersOrNull which does not support instance of Type like below. https://github.com/spring-projects/spring-framework/blob/877a3d362b6e619a09eb48a8588857b3f75b09a5/spring-web/src/main/java/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverter.java#L176

As a result of this, it encounters below error

java.lang.IllegalArgumentException: typeToken should be an instance of Class<?>, GenericArray, ParametrizedType or WildcardType, but actual type is org.springframework.core.ResolvableType$EmptyType@20eaf254 class org.springframework.core.ResolvableType$EmptyType
    at kotlinx.serialization.SerializersKt__SerializersJvmKt.serializerByJavaTypeImpl$SerializersKt__SerializersJvmKt(SerializersJvm.kt:127) ~[kotlinx-serialization-core-jvm-1.4.0.jar:1.4.0]
    at kotlinx.serialization.SerializersKt__SerializersJvmKt.serializerOrNull(SerializersJvm.kt:76) ~[kotlinx-serialization-core-jvm-1.4.0.jar:1.4.0]
    at kotlinx.serialization.SerializersKt.serializerOrNull(Unknown Source) ~[kotlinx-serialization-core-jvm-1.4.0.jar:1.4.0]
    at kotlinx.serialization.SerializersKt__SerializersJvmKt.serializerOrNull(SerializersJvm.kt:43) ~[kotlinx-serialization-core-jvm-1.4.0.jar:1.4.0]
    at kotlinx.serialization.SerializersKt.serializerOrNull(Unknown Source) ~[kotlinx-serialization-core-jvm-1.4.0.jar:1.4.0]
    at org.springframework.http.converter.json.KotlinSerializationJsonHttpMessageConverter.serializer(KotlinSerializationJsonHttpMessageConverter.java:176) ~[spring-web-6.0.0-M6.jar:6.0.0-M6]
    at org.springframework.http.converter.json.KotlinSerializationJsonHttpMessageConverter.canWrite(KotlinSerializationJsonHttpMessageConverter.java:99) ~[spring-web-6.0.0-M6.jar:6.0.0-M6]
    at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:286) ~[spring-webmvc-6.0.0-M6.jar:6.0.0-M6]
    at org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor.handleReturnValue(HttpEntityMethodProcessor.java:238) ~[spring-webmvc-6.0.0-M6.jar:6.0.0-M6]

Comment From: bellatoris

duplicated with https://github.com/spring-projects/spring-framework/issues/29192