[BUG][typescript-fetch] Generated code drops some values when using oneOf

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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

The typescript-fetch generator generates code for models that drops values that should not be dropped when using oneOf with type: object, {type: array, items: {nullable: true}} or {type: array, items: {type: object}}. Take a look at the following OpenAPI declaration:

openapi: "3.0.2"
info:
  title: Foo
  version: 1.0.0

paths:
  /foo:
    post:
      requestBody:
        content:
          application/json:
            schema:
              oneOf:
                - type: object
                - type: array
                  items:
                    nullable: true
      responses:
        204:
          description: "Ok"

  /bar:
    post:
      requestBody:
        content:
          application/json:
            schema:
              oneOf:
                - type: object
                - type: array
                  items:
                    type: object
      responses:
        204:
          description: "Ok"

It generates

// models.FooPostRequest.ts
/* tslint:disable */
/* eslint-disable */
/**
 * Foo
 * No description provided (generated by Openapi Generator https://github.com/openapitools/openapi-generator)
 *
 * The version of the OpenAPI document: 1.0.0
 * 
 *
 * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
 * https://openapi-generator.tech
 * Do not edit the class manually.
 */

/**
 * @type FooPostRequest
 * 
 * @export
 */
export type FooPostRequest = Array<any | null> | object;

export function FooPostRequestFromJSON(json: any): FooPostRequest {
    return FooPostRequestFromJSONTyped(json, false);
}

export function FooPostRequestFromJSONTyped(json: any, ignoreDiscriminator: boolean): FooPostRequest {
    if (json == null) {
        return json;
    }
    return {} as any;
}

export function FooPostRequestToJSON(json: any): any {
    return FooPostRequestToJSONTyped(json, false);
}

export function FooPostRequestToJSONTyped(value?: FooPostRequest | null, ignoreDiscriminator: boolean = false): any {
    if (value == null) {
        return value;
    }
    return {};
}

// BarPostRequest.ts
/* tslint:disable */
/* eslint-disable */
/**
 * Foo
 * No description provided (generated by Openapi Generator https://github.com/openapitools/openapi-generator)
 *
 * The version of the OpenAPI document: 1.0.0
 * 
 *
 * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
 * https://openapi-generator.tech
 * Do not edit the class manually.
 */

/**
 * @type BarPostRequest
 * 
 * @export
 */
export type BarPostRequest = Array<object> | object;

export function BarPostRequestFromJSON(json: any): BarPostRequest {
    return BarPostRequestFromJSONTyped(json, false);
}

export function BarPostRequestFromJSONTyped(json: any, ignoreDiscriminator: boolean): BarPostRequest {
    if (json == null) {
        return json;
    }
    return {} as any;
}

export function BarPostRequestToJSON(json: any): any {
    return BarPostRequestToJSONTyped(json, false);
}

export function BarPostRequestToJSONTyped(value?: BarPostRequest | null, ignoreDiscriminator: boolean = false): any {
    if (value == null) {
        return value;
    }
    return {};
}

The RequestFromJSONTyped and RequestToJSONTyped functions do not return the desired array or object values and instead replace them with an empty object.

openapi-generator version

7.15

Generation Details

typescript-fetch generator with default settings.

Steps to reproduce

Take the OpenAPI definition from above and run the OpenAPI generator with the typescript-fetch generator.

Related issues/PRs

Suggest a fix

I believe the modelOneOf.mustache template is missing some cases. I will try to open a pull request.

Start with the typescript-fetch generator and its modelOneOf.mustache template, then reproduce the issue using the OpenAPI definition and default settings described here. Confirm that the generated RequestFromJSONTyped and RequestToJSONTyped functions preserve the array and object values instead of replacing them with empty objects.