Description

insidewhy

opened on Dec 27, 2020

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Bug Report

🔎 Search Terms

key remapping tuples mapped types

🕗 Version & Regression Information

Typescript 4.1

• I was unable to test this on prior versions because Typescript 4.1 introduced key remapping

⏯ Playground Link

Playground link with relevant code

💻 Code

type NoBoolean<K, T> = T extends boolean ? never : K

type FilterBooleans<T> = {
  [K in keyof T as NoBoolean<K, T[K]>]: T[K]
}

type Filtered = FilterBooleans<[string, boolean, number, boolean]>

// should only accept ["1", 3]
const f: Filtered = ["1", true, 3, false]
type Length = Filtered["length"];
// should only accept 2, not 4
const l: Length = 4;

🙁 Actual behavior

#26063 introduced mapped type support for arrays and tuples. This breaks when using the new "key remapping" feature added to TypeScript 4.1. A record type is produced that looks like a tuple, rather than the tuple type that would be produced when not using as to remap keys.

🙂 Expected behavior

A filtered tuple type should be produced.

Activity

1. insidewhy

   changed the title [-]Remapping keys breaks mapped support for tuples[/-] [+]Key remapping breaks tuple support for mapped type[/+] on Dec 27, 2020
2. insidewhy

   changed the title [-]Key remapping breaks tuple support for mapped type[/-] [+]Key remapping breaks tuple support for mapped types[/+] on Dec 27, 2020
3. jcalz commented on Dec 27, 2020

   

   Joe Calzaretta (jcalz)

   on Dec 27, 2020

   Contributor

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   Duplicate of #40586

   I have the same issue here as there: even if you could manage to preserve a tuple type upon key renaming, I don’t see how you’d get a shortened tuple from it. It would be a sparse tuple, if anything. In your mapping, the key "2" would stay "2", right?
4. insidewhy commented on Dec 27, 2020

   

   insidewhy

   on Dec 27, 2020

   Author

   More actions

   Joe Calzaretta (@jcalz) Thanks for the info. Getting a sparse tuple wouldn't be good for many use cases I think, not unless there was a way to compact it. Filtering an array is possible but not without lots of CPU and a size limit 😭
5. insidewhy

   closed this as completedon Dec 27, 2020
6. microsoft

   locked as resolved and limited conversation to collaborators on Oct 21, 2025

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