[BUG][KOTLIN] Generated kotlin code for array of binary items fails to compile

[grEvenX]

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?

Description

When using an OpenAPI schema that describes a file upload supporting multiple files, the generated kotlin code fails to compile.
This started after the 7.0.0 release (the kotlin code compiles successfully in 6.6.0, but I suspect it only supports a single file).

openapi-generator version

7.4.0 (issue introduced in 7.0.0)

OpenAPI declaration file content or url

testcase.yaml:

openapi: 3.0.0
info:
  version: "1.0.0"
  title: TestAPI
servers:
  - url: https://api.example.com/
    description: API endpoint
paths:
  /file-upload:
    post:
      operationId: uploadFile
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                # The property name 'file' will be used for all files.
                file:
                  type: array
                  items:
                    type: string
                    format: binary
      responses:
        '204':
          description: File uploaded

testconfig.json:

{
    "artifactId": "example-client",
    "artifactVersion": "1.0.0",
    "groupId": "com.example",
    "packageName": "com.example.kotlin.api",
    "enumPropertyNaming": "UPPERCASE",
    "library": "multiplatform",
    "dateLibrary": "string",
    "omitGradlePluginVersions": false
}

Generation Details

docker run --rm -v "${PWD}:/local" openapitools/openapi-generator-cli:v7.4.0 generate \
    -i /local/testcase.yaml \
    -g kotlin \
    -c /local/testconfig.json \
    -o /local/generated

Steps to reproduce

• Create the testcase.yaml and testconfig.json as described in this issue.
• Run the code generation, then run ./gradlw clean assemble to try to build the kotlin client.

The following error is reported:

> Task :compileCommonMainKotlinMetadata FAILED
e: file:///***/generated_test/src/commonMain/kotlin/com/example/kotlin/api/apis/DefaultApi.kt:56:23 Unresolved reference. None of the following candidates is applicable because of receiver type mismatch: 
public inline fun <T, C : Iterable<TypeVariable(T)>> TypeVariable(C).onEach(action: (TypeVariable(T)) -> Unit): TypeVariable(C) defined in kotlin.collections
public inline fun <K, V, M : Map<out TypeVariable(K), TypeVariable(V)>> TypeVariable(M).onEach(action: (Map.Entry<TypeVariable(K), TypeVariable(V)>) -> Unit): TypeVariable(M) defined in kotlin.collections
public inline fun <S : CharSequence> TypeVariable(S).onEach(action: (Char) -> Unit): TypeVariable(S) defined in kotlin.text
public inline fun <T> Array<out TypeVariable(T)>.onEach(action: (TypeVariable(T)) -> Unit): Array<out TypeVariable(T)> defined in kotlin.collections
public inline fun BooleanArray.onEach(action: (Boolean) -> Unit): BooleanArray defined in kotlin.collections
public inline fun ByteArray.onEach(action: (Byte) -> Unit): ByteArray defined in kotlin.collections
public inline fun CharArray.onEach(action: (Char) -> Unit): CharArray defined in kotlin.collections
public inline fun DoubleArray.onEach(action: (Double) -> Unit): DoubleArray defined in kotlin.collections
public inline fun FloatArray.onEach(action: (Float) -> Unit): FloatArray defined in kotlin.collections
public inline fun IntArray.onEach(action: (Int) -> Unit): IntArray defined in kotlin.collections
public inline fun LongArray.onEach(action: (Long) -> Unit): LongArray defined in kotlin.collections
public inline fun ShortArray.onEach(action: (Short) -> Unit): ShortArray defined in kotlin.collections
public inline fun UByteArray.onEach(action: (UByte) -> Unit): UByteArray defined in kotlin.collections
public inline fun UIntArray.onEach(action: (UInt) -> Unit): UIntArray defined in kotlin.collections
public inline fun ULongArray.onEach(action: (ULong) -> Unit): ULongArray defined in kotlin.collections
public inline fun UShortArray.onEach(action: (UShort) -> Unit): UShortArray defined in kotlin.collections
public fun <T> Sequence<TypeVariable(T)>.onEach(action: (TypeVariable(T)) -> Unit): Sequence<TypeVariable(T)> defined in kotlin.sequences
e: file:///***/generated_test/src/commonMain/kotlin/com/example/kotlin/api/apis/DefaultApi.kt:57:38 Unresolved reference: it

The generated API method looks like this (onEach is not supported since the file variable is not a collection):

    open suspend fun uploadFile(file: io.ktor.client.request.forms.InputProvider? = null): HttpResponse<Unit> {

        val localVariableAuthNames = listOf<String>()

        val localVariableBody = 
            formData {
                file?.onEach {
                    append("file[]", it)
                }
            }
            ...

I'm a bit unsure what the expected output is, but the output when using v6.6.0 was this for the formData:

val localVariableBody = 
            formData {
                file?.apply { append("file", file) }
            }

I would guess this only supports a single file, though, unless InputProvider can hold multiple files.

Related issues/PRs

Suggest a fix

[martin-mfg]

This problem occurs since #16329.
(cc @YusukeOba)