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Different behavior when using a type as is, or as a type alias #57062
Description
🔎 Search Terms
type alias
extends array
different behavior
🕗 Version & Regression Information
This was introduced by version 4.2, and continues as of today (version 5.3).
⏯ Playground Link
💻 Code
type A<T> = {a: T extends true[] ? true : false};
type B = A<true[]>; // {a: true}
type C = A<boolean[]>; // {a: false}
type D = A<true[]> extends C ? true : false; // `false`, which is correct
type E = A<true[]> extends A<boolean[]> ? true : false; // `true`, which is incorrect🙁 Actual behavior
The types D and E have different values.
🙂 Expected behavior
The types D and E should have the same value since they are identical, except for one using a type alias.
Activity
[-]Different behavior when using a type as is, or as a declared `type`[/-][+]Different behavior when using a type as is, or as a type alias[/+]fatcerberus commentedon Jan 15, 2024
Looks like a variance measurement issue (
Ais erroneously measured as covariant)ehmicky commentedon Jan 15, 2024
Thanks Bruce Pascoe (@fatcerberus). I can confirm this,as the following code works:
whzx5byb commentedon Jan 15, 2024
Duplicate of #48070
ehmicky commentedon Jan 15, 2024
Thanks whzx5byb and sorry for the duplicate. Closing in favor of #48070.