Conditional type only works if I inline it about flow HOT 5 CLOSED

I have a smaller repro here.

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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.

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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

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@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>>);

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@SamChou19815 Thanks for the fix!

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