[BUG] Kotlin-generator uses immutable empty initializers when using modelMutable

[mkoertgen]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

openapi-generator version

7.12.0

OpenAPI declaration file content or url

Generation Details

Using the latest gradle-plugin, the kotlin-client-generator and mutable models

Steps to reproduce

Server: Simple Spring Boot WebMvc app using SpringDoc. Having a Response object like

@Schema(
            description = "Membership roles",
            example = """["ADMIN", "USER"]""",
            defaultValue = "[]"
        )
        @JsonView(ApiView.Get::class)
        val roles: Set<String> = emptySet(),

generates a schema like

  "roles": {
      "uniqueItems": true,
      "type": "array",
      "description": "Membership roles",
      "example": ["ADMIN", "USER_ADMIN"],
      "items": {
          "type": "string",
          "description": "Membership roles",
          "example": "[\"ADMIN\",\"USER\"]",
          "default": "[]"
      },
      "default": []
  },

Client: Using this schema with the kotlin-client-generator together with modelMutable=true will correctly use a MutableSet yet the defaultValue-initializer will still point to an immutable setOf()

    /* Membership roles */
    @get:JsonProperty("roles")
    var roles: kotlin.collections.MutableSet<kotlin.String>? = setOf(),

Related issues/PRs

The mutableModel-features has been addressed in #11088

• [REQ][Kotlin] Use mutable container types when 'modelMutable' is enabled #11088
• [BUG][Kotlin] Wrong default value is generated if the field type is BigDecimal #10866

Suggest a fix

As a workaround, one can skip the default-Value annotation and agree on not skipping null/empty-values in json-serialization between server and client.