[BUG][Typescript] Generated code with `oneOf` does not properly check the dto type

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

Description

Given the OpenApi schema below, we have Dto with payload field, which has a type of oneOf of Payload1, Payload2 and Payload3. Each of payload types has a property type with type of enum with only one value.

When we generate client code, we get this code in models/DtoPayload.ts file:

export function DtoPayloadToJSON(value?: DtoPayload | null): any {
    if (value == null) {
        return value;
    }

    if (instanceOfPayload1(value)) {
        return Payload1ToJSON(value as Payload1);
    }
    if (instanceOfPayload2(value)) {
        return Payload2ToJSON(value as Payload2);
    }
    if (instanceOfPayload3(value)) {
        return Payload3ToJSON(value as Payload3);
    }

    return {};
}

and code for each if check:

models/Payload1.ts

export function instanceOfPayload1(value: object): boolean {
    if (!('type' in value)) return false;
    if (!('barcode' in value)) return false;
    return true;
}

models/Payload2.ts

export function instanceOfPayload2(value: object): boolean {
    if (!('type' in value)) return false;
    return true;
}

models/Payload3.ts

export function instanceOfPayload3(value: object): boolean {
    if (!('type' in value)) return false;
    if (!('barcode' in value)) return false;
    return true;
}

since functions instanceOfPayload1 and instanceOfPayload3 are identical, generated code treats Payload3 as Payload1, which causes problems when converting plain objects to dto's.

openapi-generator version

7.5.0

OpenAPI declaration file content or url

{
  "openapi": "3.0.0",
  "paths": {
    "/endpoint": {
      "get": {
        "operationId": "endpoint",
        "summary": "endpoint",
        "responses": {
          "200": {
            "description": "",
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/Dto"
                }
              }
            }
          }
        },
        "security": [
          {
            "access-token": []
          }
        ]
      }
    }
  },
  "info": {
    "title": "API",
    "description": "API",
    "version": "1.0",
    "contact": {},
    "license": {
      "name": "UNLICENSED",
      "url": "UNLICENSED"
    }
  },
  "tags": [],
  "servers": [
  ],
  "components": {
    "securitySchemes": {
      "access-token": {
        "scheme": "bearer",
        "bearerFormat": "JWT",
        "type": "http",
        "description": "JWT Authorization header using the Bearer scheme."
      }
    },
    "schemas": {
      "Payload3": {
        "type": "object",
        "properties": {
          "type": {
            "type": "string",
            "enum": [
              "AGE_VERIFICATION"
            ]
          },
          "barcode": {
            "type": "string"
          },
          "requiredAge": {
            "type": "number"
          }
        },
        "required": [
          "type",
          "barcode"
        ]
      },
      "Payload2": {
        "type": "object",
        "properties": {
          "type": {
            "type": "string",
            "enum": [
              "ASSISTANCE_REQUESTED"
            ]
          }
        },
        "required": [
          "type"
        ]
      },
      "Payload1": {
        "type": "object",
        "properties": {
          "type": {
            "type": "string",
            "enum": [
              "PARTIAL_RESCAN"
            ]
          },
          "barcode": {
            "type": "string"
          }
        },
        "required": [
          "type",
          "barcode"
        ]
      },
      "Dto": {
        "type": "object",
        "properties": {
          "payload": {
            "description": "Payload for the assistance request",
            "oneOf": [
              {
                "$ref": "#/components/schemas/Payload1"
              },
              {
                "$ref": "#/components/schemas/Payload2"
              },
              {
                "$ref": "#/components/schemas/Payload3"
              }
            ]
          }
        },
        "required": [
          "payload"
        ]
      }
    }
  }
}

Generation Details

npx -y @openapitools/openapi-generator-cli generate -g typescript-fetch
--additional-properties=importFileExtension=".js" --skip-validate-spec
-i ./test.json
-o src/api-client

Steps to reproduce

1. make directory: test
2. cd into directory test
3. create file test.json and copy and paste provided api schema above.
4. run provided in Generation Details command in cli

Related issues/PRs

Suggest a fix

We want the generator to also take into account types of fields, when checking if object is instance of another object.

for example:

export function instanceOfPayload1(value: object): boolean {
    if (!('type' in value && value.type === Object.values(Payload1TypeEnum)[0])) return false;
    if (!('barcode' in value)) return false;
    return true;
}

Start by reproducing the issue with the provided test.json and typescript-fetch generation command. Inspect the generated models/DtoPayload.ts and the instanceOfPayload1, instanceOfPayload2, and instanceOfPayload3 functions in the generated Payload files. Done means the generated checks distinguish Payload1 and Payload3 using their field values, with coverage or verification for the supplied oneOf schema.