watch should type old value as possibly undefined

[zippaaa]

Docs: https://vue-composition-api-rfc.netlify.com/api.html#watch (Watching Multiple Sources)
Test:

setup() {
    const test = ref({a: 0, b: 0}); //reactive({a: 0, b: 0});
    watch(
        () => [test.a, test.b],
        ([newA, newB], [oldA, oldB]) => console.log(newA, oldA, newB, oldB)
    );
}

Result

Test:

setup() {
    const test = reactive({a: 0, b: 0});
    watch(
        () => [test.a, test.b],
        (newValues, oldValues) => console.log(newValues, oldValues)
    );
}

Result:
Array[0, 0], undefined

[posva]

On the first run, the old value is undefined, so the error makes sense.
However, the typings are not right (vuejs/core#719) and they should follow the same fix as vuejs/core@c6a9787

[colinscz]

@posva do I get it right, that more or less the same fixes as you mentioned for vue-next have to be applied to the corresponding files here?

I would like to work on this. Let me know if this is still ready to take :)

[posva]

@cnschwarz This is still up! I believe the fix is indeed very similar to the fix on vue-next

[colinscz]

@posva awesome, that is what I thought. I started with some of the changes yesterday. I will reach out if I have specific questions :)

[pikax]

Can you check if this is still a problem on ^0.6.1?

[antfu]

Closing this for now. If you still facing the issue, feel free to reopen it.