Narrowing generic union function argument doesn't narrow return type based on generic

[zachstence]

Acknowledgement

• I acknowledge that issues using this template may be closed without further explanation at the maintainer's discretion.

Comment

I am using the generic "Other" template for this issue because I am not sure if it is a bug, I am misunderstanding something, or it would be a feature request. I would be happy to close and reopen with another template if necessary!

Reproduction:
https://www.typescriptlang.org/play?ts=5.4.0-dev.20240126#code/MYewdgzgLgBAZjAvDAPAFRgUwB5U2AEwhgHIBDEmAH1ICMSA+ACigC4Y0BKdjHPQ4uUoB+UtkrsSAT0qIGMAN4BYAFAwYASwQskiZEM4wATpigBXI2DElV6k+cukZqgL6qgA

const f = <T extends 'a' | 'b'>(t: T): T extends 'a' ? 'x' : 'y' => {
  if (t === 'a') return 'x' // Type '"x"' is not assignable to type 'T extends "a" ? "x" : "y"'. (2322)
  return 'y' // Type '"y"' is not assignable to type 'T extends "a" ? "x" : "y"'. (2322)
}

Expected behavior:
Compiles without error, and function return type is narrowed by control flow in function body checking type of argument

Actual behavior:
Type '"x"' is not assignable to type 'T extends "a" ? "x" : "y"'.(2322)
Type '"y"' is not assignable to type 'T extends "a" ? "x" : "y"'.(2322)

Notes:
This is a minimal reproduction of a problem I came across when trying to do something similar with class generics and fields. I would be happy to provide some more detail about why this would be useful if necessary.

[zachstence]

I have also taken a look at #7294 and #13995, but I think this issue is different from those. However I may be overlooking something inherently wrong with the code, and would love some insight here.

[MartinJohns]

Duplicate of #33912.

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.