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Broken double inference step in variadic tuple types when rest elements come into playΒ #52017
Description
Bug Report
π Search Terms
variadic tuple type, rest element, infer, unknown[]
It might be related to #51138 and #51157
π Version & Regression Information
This is the behavior in every version I tried, nightly included.
β― Playground Link
Playground link with relevant code
π» Code
The double inference step is needed to preserve labels of both the first element and the rest.
type HEAD_TAIL<T extends readonly any[]> =
T extends readonly [any?, ...infer TAIL]
?
T extends readonly [...infer HEAD, ...TAIL]
? {
HEAD: HEAD;
TAIL: TAIL;
}
: never
: never;π Actual behavior
The following is inferred as { HEAD: unknown[]; TAIL: [b: 2, ...c: 3[]]; }
type test3 = HEAD_TAIL<[a:1, b:2, ...c: 3[]]>π Expected behavior
The expected type is { HEAD: [a:1]; TAIL: [b: 2, ...c: 3[]]; }.
Activity
jfet97 commentedon Dec 25, 2022
Pinging Mateusz BurzyΕski (@Andarist) as requested from him.
Andarist commentedon Dec 26, 2022
Potentially a simpler version (with a single step) of this conditional type could work here as well but it doesn't work either:
While #51157 improves the situation a little bit for the
extends [any?]part, it doesn't handle 2 (or more) variadic elements in the target type here. There is no code to support such a case there - which kinda makes sense since this didn't make a lot of sense prior toinfer extendssyntax and that is a fairly new addition.Fixing this would require rewriting the whole part of
inferFromObjectTypesthat relates to tuples. Right now the logic there tries to "pattern match" some specific situations but withinfer extendswe could support a dynamic number of variadic elements (if they are constrained withextends).For similar reasons your original repro here doesn't work - such a mix of a variadic and rest elements is just not handled at all by the current logic.
I'll keep thinking about how to rewrite this whole bit to handle both cases - if you'd like to brainstorm how the improved algorithm could work, my DMs are open π
fatcerberus commentedon Dec 26, 2022
Isn't
[ ...infer HEAD, ...infer TAIL ]ambiguous since the input tuple could be any length and the choice of where to split is arbitrary?Andarist commentedon Dec 26, 2022
in Andrea Simone Costa (@jfet97)βs example
TAILis being simply spread, in my exampleHEADhas a fixed-size constraint so it isnβt ambiguous