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

[micolous]

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:

openapi-generator/samples/client/petstore/typescript-fetch/builds/default/models/User.ts

Lines 100 to 107 in 587fcff

	export function UserToJSON(json: any): User {
	return UserToJSONTyped(json, false);
	}
	export function UserToJSONTyped(value?: User | null, ignoreDiscriminator: boolean = false): any {
	if (value == null) {
	return value;
	}

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 (587fcff)

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 #19524

Suggest a fix

Change this:

openapi-generator/modules/openapi-generator/src/main/resources/typescript-fetch/modelOneOf.mustache

Lines 44 to 46 in 587fcff

	export function {{classname}}ToJSON(json: any): any {
	return {{classname}}ToJSONTyped(json, false);
	}

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:

openapi-generator/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache

Lines 111 to 113 in 587fcff

	export function {{classname}}ToJSON(json: any): {{classname}} {
	return {{classname}}ToJSONTyped(json, false);
	}

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

openapi-generator/modules/openapi-generator/src/main/resources/typescript-fetch/modelEnum.mustache

Lines 22 to 28 in 587fcff

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