[BUG] [kotlin-client / okhttp] Dollar sign in query parameter names generated invalid code

Created by: kunzese

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? See Suggest a fix
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

We consume a third-party API that has several query parameters that start with a dollar sign ($). Creating a kotlin-client with the okhttp lib results in an Unresolved reference: expand error, because the dollar sign is not escaped and is interpreted as a string template.

openapi-generator version

5.2.1

OpenAPI declaration file content or url

openapi: "3.0.0"
info:
  version: "1.0.0"
  title: "Demo"
paths:
  /:
    get:
      operationId: demo
      parameters:
        - in: query
          name: $expand
          schema:
            type: string
      responses:
        '200':
          description: OK

Generation Details

Via gradle plugin

openApiGenerate {
    generatorName = "kotlin"
    inputSpec = "$rootDir/specs/api.yaml"
}

Suggest a fix

Escape dollar signs in string literals.

Unresolved reference: expand

    fun demoRequestConfig(dollarExpand: kotlin.String?) : RequestConfig<Unit> {
        val localVariableBody = null
        val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
            .apply {
                if (dollarExpand != null) {
                    put("$expand", listOf(dollarExpand.toString()))
                }
            }
        ...

Fixed:

    fun demoRequestConfig(dollarExpand: kotlin.String?) : RequestConfig<Unit> {
        val localVariableBody = null
        val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
            .apply {
                if (dollarExpand != null) {
                    put("\$expand", listOf(dollarExpand.toString()))
                }
            }
        ...