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Inferring rest of array in a conditional results in unknown[] #45281
Description
Bug Report
🔎 Search Terms
infer conditional array rest unknown
🕗 Version & Regression Information
- I was unable to test this on prior versions than 4.1 because it uses template literal types added in 4.1
⏯ Playground Link
Playground link with relevant code
💻 Code
Using code from the #40336 description restricted to strings only:
type Join<S extends string[], D extends string> =
S extends [] ? '' :
S extends [string] ? `${S[0]}` :
S extends [string, ...infer R] ? `${S[0]}${D}${Join<R, D>}` :
string;🙁 Actual behavior
R is inferred as being unknown[].
🙂 Expected behavior
R should be inferred as being string[].
Activity
[-]Infering rest of array in a conditional results in `unknown[]`[/-][+]Inferring rest of array in a conditional results in `unknown[]`[/+]cypherfunc commentedon Mar 31, 2022
Has there been any movement on this?
The example can be simplified further, if it helps:
I've been able to get around this by adding an extra conditional check, but it's awkward/confusing in the middle of more complex types:
nag commentedon Jun 6, 2022
Not only the rest of the array is misinferred, but any element as well:
cypherfunc commentedon Jun 8, 2022
For any future folks, this is now easily solved with constraints in 4.7:
https://devblogs.microsoft.com/typescript/announcing-typescript-4-7/#extends-constraints-on-infer-type-variables