[BUG][Typescript-fetch]: Conflicting API response signatures where isResponseOptional is true

[danowensdev]

Description

When a schema contains an empty 20x response in addition to one with a body, the function defined in the API interface does not match the signature of the implementing function.

This regression was introduced as a side-effect of a bugfix for this issue: #20248

openapi-generator version

7.10.0
This is a regression introduced in 7.3.0 by #17635; while the signature was correctly changed in the implementing function, it was not changed in the API interface.

OpenAPI declaration file content or url

Example schema leading to bug:

/pets/events/:
  post:
    summary: A webhook for pet-related events
    operationId: publishPetEvent
    requestBody:
      $ref: '#/components/requestBodies/PetEvent'
    responses:
      '200':
        description: The response for the handshake event
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/HandshakeResponse'
      '204':
        description: The event has been handled
      default:
        $ref: '#/components/responses/APIError'
    tags:
      - Pets

This produces output like

export interface PetsApiInterface {
  ...
  publishPetEvent(
    requestParameters: PublishPetEventRequest,
    initOverrides?: RequestInit | runtime.InitOverrideFunction
  ): Promise<HandshakeResponse>;
}

export class PetsApi extends runtime.BaseAPI implements PetsApiInterface {
  async publishPetEvent(
    requestParameters: PublishPetEventRequest,
    initOverrides?: RequestInit | runtime.InitOverrideFunction
  ): Promise<HandshakeResponse | null | undefined> {
    const response = await this.publishPetEventRaw(requestParameters, initOverrides);
    switch (response.raw.status) {
      case 200:
        return await response.value();
      case 204:
        return null;
      default:
        return await response.value();
    }
  }
}

As can be seen, the return signature of the implementing function (Promise<HandshakeResponse | null | undefined>) does not match the signature as defined in the interface (Promise<HandshakeResponse>).
Expected would be that both include null and undefined as the promise type.

Generation Details

-g typescript-fetch \
--type-mappings=DateTime=string,Date=string \
--additional-properties=withInterfaces=true,stringEnums=true

Steps to reproduce

1. Create an endpoint with a 200 response, including a JSON body, and a 204 response with no body.
2. Run generation with typescript-fetch
3. Observe error caused by mismatched signature

Suggest a fix

The fix would be to make the same change to the response signature in the interface as to its implementation. I will submit and link a pull request to do this.