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Variadic element of tuple-like intersection types are spreading incorrectly #40945
Description
TypeScript Version: 4.0.3
Search Terms: variadic tuple, variadic element, intersection type, tuple-like type
Code
type T1 = [...unknown[] & []]; // never[]
type T2 = [...number[] & [1, 2, 3]]; // (3 | 1 | 2)[]Expected behavior: for T1, I expected it to be [](empty tuple type) and for T2, I expected it to be [1, 2, 3].
Actual behavior: Both T1 and T2 became Array rather than tuple somehow. (T1 is deduced to never[], T2 is deduced to (3 | 1 | 2)[])
detailed explanation
In some perspective, unknown could be recognized as type of all types. Therefore, [unknown] is type of all type-level 1-tuples and intuitively, unknown[] is type of all type-level n-tuples.
So [] <: unknown[] is true (i.e. all empty type-level tuple is subtype of all type-level n-tuples) and you can check it simply with this code:
type M = [] & unknown[] extends [] ? [] extends [] & unknown[] ? true : false : false; // true
However, in spite of the fact that [] is subtype of unknown[] therefore [] & unknown[] is actually [], variadic element of [] & unknown[] somehow spreads as never[], which is incorrect and unintuitive.
Playground Link: Playground
Activity
andrewbranch commentedon Oct 7, 2020
Unintuitive, yes; not as good as we could do, yes; but “incorrect” is somewhat debatable. (As an aside, this issue led to a great conversation about whether
never[]is conceptually the same as[], despite specific differences in their assignability.) At any rate, sincenever[]is not assignable to[], that’s a good argument forT1being “incorrect,” but(3 | 1 | 2)[]is clearly a valid representation of[1, 2, 3]; it’s just not a very good one 😄Out of curiosity, since there are 4 👍 on an issue that seems somewhat arcane, is this coming up in practice somewhere?
ENvironmentSet commentedon Oct 7, 2020
I was building type-level programming library since TS 4.1 finally added recursive conditional type alias. But after few hours, I encountered this issue and talked with others in twitter for a few days. That 👍seems to be added by people I know or discussed with me about this.
ENvironmentSet commentedon Oct 8, 2020
I agree on that T1 is not being incorrect, sorry for misusing word. However, I think we all agree on that ( 3 | 2 | 1 )[] is not a proper or useful typing at all at this point since some important informations are discarded and it might cause some unexpected problems. 🙂
(Sorry for loose formatting, I wrote this comment with my mobile phone...)
sethveale commentedon Oct 23, 2020
Similar scenario for me. I was trying to map nested tuple types.
mkantor commentedon Mar 11, 2022
This also becomes an array type, even though both sides of the intersection were tuples to begin with:
geoffreytools commentedon Mar 11, 2022
Actually, this is also broken
[...[1] & [unknown]]I had a similar "issue" when trying to get a tuple back from a tuple augmented with other properties.
Investigation led me to the discovery that even
[...([1,2,3] & unknown[])]was lossyYou will find bellow my different attempts at getting the tuple back. Explicitly iterating over the numeric keys worked but I was surprised I had to go this far.