[BUG][typescript-fetch] Further union issues

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 issuue still exists?
• Have you search 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

With https://github.com/OpenAPITools/openapi-generator/pull/6789 merged I regenerated my client and unfortunately there are still some union-related errors.

There are multiple locations with invalid code generated for handling union values:

        return new runtime.JSONApiResponse(response, (jsonValue) => jsonValue.map(Pet | Cat | DogFromJSON));

        const response = await this.request({
            path: `/pets`,
            method: 'POST',
            headers: headerParameters,
            query: queryParameters,
            body: Pet | Cat | DogToJSON(requestParameters.petCatDog),
        });

No errors are thrown during generation but when compiling with typescript it's failing.

openapi-generator version

openapi-generator-cli-5.0.0-20200907.080402-621.jar

OpenAPI declaration file content or url

openapi: "3.0.1"
info:
  version: 1.0.0
  title: typescript union issue
paths:
  /pets:
    get:
      responses:
        "200":
          description: OK
          content:
            application/json:
              schema:
                type: array
                items:
                  oneOf:
                    - $ref: "#/components/schemas/Pet"
                    - $ref: "#/components/schemas/Cat"
                    - $ref: "#/components/schemas/Dog"
                  discriminator:
                    mapping:
                      cat: "#/components/schemas/Cat"
                      dog: "#/components/schemas/Dog"
                    propertyName: "petType"
    post:
      requestBody:
        content:
          application/json:
            schema:
              oneOf:
                - $ref: "#/components/schemas/Pet"
                - $ref: "#/components/schemas/Cat"
                - $ref: "#/components/schemas/Dog"
              discriminator:
                mapping:
                  cat: "#/components/schemas/Cat"
                  dog: "#/components/schemas/Dog"
                propertyName: "petType"
      responses:
        "201":
          description: Created
  /pets-alt:
    get:
      responses:
        "200":
          description: OK
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/PetsResponse"
components:
  schemas:
    Pet:
      type: object
      required:
        - petType
      properties:
        petType:
          type: string
        name:
          type: string
    Cat:
      type: object
      allOf:
        - $ref: "#/components/schemas/Pet"
        - type: object
          properties:
            scratching:
              type: boolean
    Dog:
      type: object
      allOf:
        - $ref: "#/components/schemas/Pet"
        - type: object
          properties:
            sniffing:
              type: boolean
    PetsResponse:
      type: object
      properties:
        pets:
          type: array
          items:
            oneOf:
              - $ref: "#/components/schemas/Pet"
              - $ref: "#/components/schemas/Cat"
              - $ref: "#/components/schemas/Dog"
            discriminator:
              mapping:
                cat: "#/components/schemas/Cat"
                dog: "#/components/schemas/Dog"
              propertyName: "petType"

Generation Details

Generator: typescript-fetch
Config:

{
  "npmName": "@company/project",
  "npmVersion": "1.0.0",
  "snapshot": false,
  "supportsES6": true,
  "typescriptThreePlus": true
}

Steps to reproduce

This issue can be reproduced by using the typescript-fetch generator with the OpenAPI declaration & config from above.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/6789
https://github.com/OpenAPITools/openapi-generator/issues/6513

Suggest a fix

Start with the typescript-fetch generator entry point and reproduce the issue using the OpenAPI declaration and config in this report. Compile the generated client and inspect the deserialization and request-body paths shown in the examples; done means the generated union handling is valid TypeScript in both cases.