Adding term with duplicate name via REST API no longer returning existing term ID

[shooper]

Reported by @coleh in the ticket that caused this change, #41370.

In WordPress 4.9, if you add a term with a duplicate name using the REST API, the 'data' element used to contain the ID of the existing term. It now just an array that only includes the HTTP status code.

Here's the output of the REST API calls in both 4.8 and 4.9:

In WordPress 4.8, returns HTTP status code 500 and this body:

{
    "code": "term_exists",
    "message": "A term with the name provided already exists with this parent.",
    "data": 2
}

In WordPress 4.9, returns HTTP status code 409 and this body:

{
    "code": "term_exists",
    "message": "A term with the name provided already exists with this parent.",
    "data": {
        "status": 409
    }
}

Note that this was my original patch. I'm happy to provide the fix for it here.

[shooper]

I don't believe it's possible to have 100% backward compatibility with 4.8.x as by changing the HTTP status code to 409, the data element in the response became an array instead of an integer. What I suggest is adding 'term_id' to the data array.

Is there a way to change the status code without using the add_data() function?

[shooper]

Patch that adds term_id to the response's data array.

[shooper]

Provided a patch that adds term_id to the REST API response. Are we OK with this approach, given that it still doesn't provide 100% backwards compatibility with the response given in WP 4.8.x ?

[swissspidy]

Duplicate of #42597.

The existing term ID is added to the error object one line above, isn‘t it?

Would you mind adding your patch to the existing ticket for this bug? Thanks!