[BUG][Kotlin/okhttp-gson] wrong handling of empty response

Created by: zoltanwork

Description

We use a legacy REST server that returns a non-empty response even if it the response type is Unit. In this case the following exception is thrown: java.lang.IllegalArgumentException: Platform class kotlin.Unit requires explicit JsonAdapter to be registered at com.squareup.moshi.ClassJsonAdapter$1.create(ClassJsonAdapter.java:76) at com.squareup.moshi.Moshi.adapter(Moshi.java:146) at com.squareup.moshi.Moshi.adapter(Moshi.java:106) at com.squareup.moshi.Moshi.adapter(Moshi.java:75) at com.squareup.moshi._MoshiKotlinExtensionsKt.adapter(-MoshiKotlinExtensions.kt:40) ...

I checked another generator (Java/apache-httpclient) to see how it handles this case. There the Java class of the expected result is checked and the code doesn't call the JSON deserializer if no response is expected.

openapi-generator version

6.0.1

Suggest a fix/enhancement

Suggestion is to change the line 251 in the src\main\resources\kotlin-client\libraries\jvm-okhttp\infrastructure\ApiClient.kt.mustache from

        if (bodyContent.isEmpty()) {

        if (typeOf<T>() == typeOf<Unit?>()) {