Response

// Both of these work
    @GET
    fun compiles2(): Response {
        return if (dto != null) dto
        else ErrResponse("baz")
    }

    @GET
    fun compiles3(): Response {
        val response = if (dto != null) dto
        else ErrResponse("baz")
        return response
    }

In 

fails()

, the compiler must first infer the type of the 

response

 variable, and since the 

RestResponse

 is (I'm assuming) a Java class, it tries to reconcile the Java generic variance with Kotlin, and determines it to have 

out

 variance.

data class RestResponse<T>(val status: Int, val entity: T) {
    companion object {
        fun <T> ok(entity: T): RestResponse<T> = RestResponse(200, entity)
        fun <T> status(status: Int, entity: T): RestResponse<T> = RestResponse(status, entity)
    }
}

RestResponse<Response>

RestResponse.ok

RestResponse.status

ok<Response>(...)

status<Response>(...)

RestResponse<Response>

RestResponse<out Response>