[BUG] [typescript-fetch] regression: modelToJSON method has incorrect type signature

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

The typescript-fetch generator builds a ModelToJSON and ModelToJSONTyped method for each model type, for example:

https://github.com/OpenAPITools/openapi-generator/blob/587fcff85c87c135a691988227fbee3216e4876b/samples/client/petstore/typescript-fetch/builds/default/models/User.ts#L100-L107

In the example above, UserToJSON calls UserToJSONTyped.

UserToJSONTyped’s signature is correct: it takes a value: User? | null, and returns an any.

UserToJSON’s signature is incorrect: it takes the same arguments and returns the same values, but it is declared as taking a json: any (rather than value: User? | null) and returning a User.

While in the simple case, User and the “JSON User” models are equivalent, this is not true if a field needs special handling when serialising or deserialising. For example, a Date field expressed as an ISO 7816 timestamp (string) or integer timestamp in JSON.

openapi-generator version

Current HEAD (587fcff85c87c135a691988227fbee3216e4876b)

OpenAPI declaration file content or url

Petstore example in openapi-generator repository

Generation Details

Visible in all of the example typescript-fetch builds in the openapi-generator repository.

Steps to reproduce

As above

Related issues/PRs

This regression seems like it was introduced in https://github.com/OpenAPITools/openapi-generator/pull/19524

Suggest a fix

Change this:

https://github.com/OpenAPITools/openapi-generator/blob/587fcff85c87c135a691988227fbee3216e4876b/modules/openapi-generator/src/main/resources/typescript-fetch/modelOneOf.mustache#L44-L46

To:

export function {{classname}}ToJSON(value?: {{classname}} | null ): any {
    return {{classname}}ToJSONTyped(value, false);
}

And change this in a similar way, copying its ToJSONTyped method signature:

https://github.com/OpenAPITools/openapi-generator/blob/587fcff85c87c135a691988227fbee3216e4876b/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache#L111-L113

modelEnum has the reverse issue: ToJSON is correct, but ToJSONTyped has the wrong type information:

https://github.com/OpenAPITools/openapi-generator/blob/587fcff85c87c135a691988227fbee3216e4876b/modules/openapi-generator/src/main/resources/typescript-fetch/modelEnum.mustache#L22-L28

Inspect modelOneOf.mustache, modelGeneric.mustache, and modelEnum.mustache in modules/openapi-generator/src/main/resources/typescript-fetch, comparing each ToJSON and ToJSONTyped signature with the generated User.ts example. Regenerate the Petstore typescript-fetch samples and confirm the signatures accept and return the appropriate model or JSON types, including special serialization cases.