Conditional type only works if I inline it

[mwiencek]

Flow version: 0.222.0

I couldn't figure out why Flow is giving an error and inferring number here. (Sorry for the all the code; I tried to minimize it as much as I could. The intent of the set function is in the comment at the bottom: supposed to be equivalent to d[0].year = 1988).

At first I thought I made a typo somewhere, but after a couple hours playing with the code (:cry:), I found out the error goes away if I inline some of the conditional types.

In case it's not clear, I only changed the following in the type parameters of the set function overloads:

-K2: KeyType2<T, K1>
+K2: PropType<T, K1> extends $ReadOnlyArray<mixed> ? number : PropType<T, K1> extends {...} ? $Keys<PropType<T, K1>

which essentially expands the KeyType2 definition in-place.

I was hoping for a way to do all of this without overloads to begin with, but alas, my best attempt did not work.

[mwiencek]

I don't have to expand it within the set type parameters; it also works if expand it in the KeyType2 definition:

-type KeyType2<+T, +K1> = KeyType<PropType<T, K1>>;
+type KeyType2<+T, +K1> = PropType<T, K1> extends $ReadOnlyArray<mixed> ? number : PropType<T, K1> extends {...} ? $Keys<PropType<T, K1>> : empty;

Edit: Also, weirdly, if I revert all the inlining and just do ('year': KeyType2<DatePeriods, 0>);, then that type-checks fine. So it works in some cases, but not in the set function.

[SamChou19815]

The behavior seems expected to me. number is coming from KeyType<DatePeriods>. Conditional type is order sensitive. If you switch the order of two conditions in KeyType, it would work

[mwiencek]

@SamChou19815 Thanks for the pointer, but not sure I follow: in the expanded version the order of the conditions is the same, isn't it?

The error message suggests that K2: KeyType2<T, K1> should be number, but K1 is KeyType<DatePeriods>, which is also number. Indeed this works fine:

type KeyType2<+T, +K1> = KeyType<PropType<T, K1>>;

('year': KeyType2<DatePeriods, KeyType<DatePeriods>>);

[SamChou19815]

I have a smaller repro here.

[mwiencek]

@SamChou19815 Thanks for the fix!