kotlin-server/ktor doesn't generate nullable types

[mplattr3]

Description

The kotlin-server/ Ktor generator generates a Path.kt file that doesn't mark optional parameters as nullable.

openapi-generator version

openapi-generator-cli/4.2.3

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  version: 0.0.1
  title: Optional Parameter Demo
paths:
  /foo:
    get:
      parameters:
        - in: query
          name: optional
          schema:
            type: integer
        - in: query
          name: mandatory
          required: true
          schema:
            type: integer
      responses:
        200:
          description: ''
          content:
            text/plain:
              schema:
                type: integer

Command line used for generation

java -jar openapi-generator-cli.jar generate -i demo.yaml -g kotlin-server -o test/

Steps to reproduce

1. Run the above command on the declaration file provided

This will generate the following template:

/**
* Optional Parameter Demo
* No description provided (generated by Openapi Generator https://github.com/openapitools/openapi-generator)
*
* The version of the OpenAPI document: 0.0.1
*
*
* NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
* https://openapi-generator.tech
* Do not edit the class manually.
*/
package org.openapitools.server

import io.ktor.locations.KtorExperimentalLocationsAPI
import io.ktor.locations.Location

object Paths {
    /**
     *
     *
     * @param mandatory
     * @param optional  (optional)
     */
    @KtorExperimentalLocationsAPI
    @Location("/foo") class fooGet(val mandatory: kotlin.Int, val optional: kotlin.Int)

}

Optional parameters are not annotated as nullable (?).

Suggest a fix/enhancement

Paths.kt.mustache should be ammended to print ? for optional parameters.