[Kotlin] Fix (de)serialization of enum classes (kotlinx serialization)

Created by: kvn-stgl

Enum class which are inside the dataClass.mustache file are missing the Serializable annotation. This leads to a wrong behavior of the serializer where the names instead of the the SerialName values are used.

You can reproduce this issue if you run the kotlin-retrofit2-kotlinx_serialization sample project. A warning like Explicit @Serializable annotation on enum class is required when @SerialName or @SerialInfo annotations are used on its members. will be thrown.

The changeset also adds the Xopt-in compiler flags for the ExperimentalSerializationApi which also leads to compiler warnings.

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• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
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@jimschubert @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m

modules/openapi-generator/src/main/resources/kotlin-client/build.gradle.mustache

@@ -114,3 +114,11 @@ dependencies {
     {{/jvm-retrofit2}}
     testCompile "io.kotlintest:kotlintest-runner-junit5:3.4.2"
 }
+{{#kotlinx_serialization}}
+
+tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).configureEach {
+    kotlinOptions {
+        freeCompilerArgs += "-Xopt-in=kotlinx.serialization.ExperimentalSerializationApi"
+    }
+}
+{{/kotlinx_serialization}}
\ No newline at end of file

modules/openapi-generator/src/main/resources/kotlin-client/data_class.mustache

@@ -70,6 +70,9 @@ import java.io.Serializable
      * {{{description}}}
      * Values: {{#allowableValues}}{{#enumVars}}{{&name}}{{^-last}},{{/-last}}{{/enumVars}}{{/allowableValues}}
      */
+    {{#kotlinx_serialization}}
+    @KSerializable
+    {{/kotlinx_serialization}}
     {{#nonPublicApi}}internal {{/nonPublicApi}}enum class {{{nameInCamelCase}}}(val value: {{^isContainer}}{{dataType}}{{/isContainer}}{{#isContainer}}kotlin.String{{/isContainer}}) {
     {{#allowableValues}}
     {{#enumVars}}

samples/client/petstore/kotlin-retrofit2-kotlinx_serialization/src/main/kotlin/org/openapitools/client/models/Order.kt

@@ -50,6 +50,7 @@ data class Order (
      * Order Status
      * Values: PLACED,APPROVED,DELIVERED
      */
+    @KSerializable
     enum class Status(val value: kotlin.String) {
         @SerialName(value = "placed") PLACED("placed"),
         @SerialName(value = "approved") APPROVED("approved"),

samples/client/petstore/kotlin-retrofit2-kotlinx_serialization/src/main/kotlin/org/openapitools/client/models/Pet.kt

@@ -52,6 +52,7 @@ data class Pet (
      * pet status in the store
      * Values: AVAILABLE,PENDING,SOLD
      */
+    @KSerializable
     enum class Status(val value: kotlin.String) {
         @SerialName(value = "available") AVAILABLE("available"),
         @SerialName(value = "pending") PENDING("pending"),

samples/client/petstore/kotlin-retrofit2-kotlinx_serialization/build.gradle

@@ -40,3 +40,9 @@ dependencies {
     compile "com.squareup.retrofit2:converter-scalars:$retrofitVersion"
     testCompile "io.kotlintest:kotlintest-runner-junit5:3.4.2"
 }
+
+tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).configureEach {
+    kotlinOptions {
+        freeCompilerArgs += "-Xopt-in=kotlinx.serialization.ExperimentalSerializationApi"
+    }
+}