[BUG] [Kotlin] [Gradle Plugin] [Android] Configuration option for mutable/immutable models behaves unexpectedly

[stephen-mojo]

Description

I am using the OpenApiGenerator via the Gradle plugin in my Kotlin Android project. I noticed an issue where the configuration value for mutable model attributes is not working as expected.

According to the documentation for the Kotlin generator, a configuration option for mutableModel can be set. The description of this option is simply, "Create mutable models" and the default value is false.
https://github.com/OpenAPITools/openapi-generator/blob/master/docs/generators/kotlin.md

I have noticed that in my particular case, setting any value results in mutable models. The lack of a value all together is what actually results in immutable models.

For example, if I set mutableModel to either true or false, the models generated are made up of all var attributes:

@Parcelize
@JsonClass(generateAdapter = true)
data class UserDto (
    @Json(name = "id")
    var id: kotlin.String,
    @Json(name = "email")
    var email: kotlin.String,
    @Json(name = "isVerified")
    var isVerified: kotlin.Boolean,
    @Json(name = "givenName")
    var givenName: kotlin.String,
    @Json(name = "familyName")
    var familyName: kotlin.String
) : Parcelable

If I leave the configuration value off completely, attributes default to val:

@Parcelize
@JsonClass(generateAdapter = true)
data class UserDto (
    @Json(name = "id")
    val id: kotlin.String,
    @Json(name = "email")
    val email: kotlin.String,
    @Json(name = "isVerified")
    val isVerified: kotlin.Boolean,
    @Json(name = "givenName")
    val givenName: kotlin.String,
    @Json(name = "familyName")
    val familyName: kotlin.String
) : Parcelable

My Configuration

build.gradle

classpath "org.openapitools:openapi-generator-gradle-plugin:5.1.0"

App-level build.gradle:

openApiGenerate {
    generatorName = "kotlin"
    inputSpec = "$projectDir/openapi-spec.json"
    ignoreFileOverride = "$projectDir/.openapi-generator-ignore"
    outputDir = "$buildDir/generated/openapi"
    apiPackage = "com.my.example.api.service"
    modelPackage = "com.my.example.api.model"
    configFile = "$projectDir/openapi-generator-config.json"
}

openapi-generator-config.json

{
  "apiSuffix": "Service",
  "collectionType": "list",
  "enumPropertyNaming": "UPPERCASE",
  "library": "jvm-retrofit2",
  "modelMutable": "false",
  "moshiCodeGen": "true",
  "packageName": "com.my.example.api",
  "parcelizeModels": "true",
  "sortModelPropertiesByRequiredFlag": "true",
  "sortParamsByRequiredFlag": "true",
  "useCoroutines": "true",
  "typeMappings": {
    "java.net.URI": "kotlin.String"
  }
}

Related Tickets

#4115
#3803