[BUG] Uppercase enum field causes kotlin code generation to fail compile

Description

If you have an embedded enum such as the following:

          "OS": {
            "type": "string",
            "description": "the device operating system",
            "example": "IOS",
            "enum": [
              "IOS",
              "ANDROID",
              "WATCH_OS"
            ]
          },

The kotlin code generator will generate both a field and inner class name of "OS". This causes a compilation error, since it can't have both an inner class and a field with the same name.

Conflicting declarations: public final enum class OS : Enum<DeviceSpecificDetails.OS>, public final val OS: DeviceSpecificDetails.OS?

@JsonClass(generateAdapter = true)

data class DeviceSpecificDetails (
    /* the device operating system */
    @Json(name = "OS")
    val OS: DeviceSpecificDetails.OS? = null,
) {
    /**
     * the device operating system
     *
     * Values: IOS,ANDROID,WATCH_OS
     */
    enum class OS(val value: kotlin.String) {
        @Json(name = "IOS") IOS("IOS"),
        @Json(name = "ANDROID") ANDROID("ANDROID"),
        @Json(name = "WATCH_OS") WATCH_OS("WATCH_OS");
    }

openapi-generator version

6.2.1

Generation Details

We are using the following configuration to generate our code:

    openApiGenerate {
        generatorName.set("kotlin")
        inputSpec.set(extension.inputSpec)
        outputDir.set(extension.outputDirPath)
        packageName.set("${extension.rootPackageName}")
        apiPackage.set("${extension.rootPackageName}.api")
        modelPackage.set("${extension.rootPackageName}.models")
        invokerPackage.set("${extension.rootPackageName}.client")
        library.set("jvm-retrofit2")
        configOptions.set(
            mapOf(
                "useCoroutines" to "true",
                "artifactId" to project.name,
                "groupId" to project.group.toString(),
                "artifactVersion" to project.version.toString(),
                "serializationLibrary" to "moshi",
                "moshiCodeGen" to "true",
                "dateLibrary" to "threetenbp",
                "enumPropertyNaming" to "UPPERCASE"
            )
        )
    }

Steps to reproduce

Attempt to generate and then compile the generated code.

Suggest a fix

Java code generation appends an "Enum" to any enum class that is generated (at least for inner classes). I suggest that perhaps the kotlin generate needs to do the same thing, or at least give the user a configuration option for how to handle this sort of case.

Reproduce Kotlin generation from the embedded OS enum example using the listed Moshi and enumPropertyNaming settings, then inspect the generated DeviceSpecificDetails declaration. The payload names no source file or test; done means the generated Kotlin code no longer has conflicting field and inner enum declarations and compiles successfully.