Generic function return value not fully inferred through generic argument

[m-dressler]

🔎 Search Terms

"typescript generic constraint return type", "typescript generic constraint return type error", "ts 2322", "typescript 2322 constraint return type"

🕗 Version & Regression Information

• Tested on nightly & 5.1.6

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.3.0-dev.20230828#code/MYewdgzgLgBAhjAvDAPAFRgUwB5U2AEwhgCMQQAbTOMGAHxjAFcKKA+ACjgCcBzALhhoAlIIw48hYs1YwA-IyYBbEpm4xB0bgEswvJGxgBvAFAwY2gGYwufJImQyKwmN0xQm3WgAYA3Gdd3T1oAIhD-AF9fIA

💻 Code

const a = <T extends boolean | null>(arg: T): T extends null ? number : string => {
  if (arg === null) return 0;
  return "";
};

🙁 Actual behavior

Caused a typescript error Type 'number' is not assignable to type 'T extends null ? number : string'.(2322)

🙂 Expected behavior

No error as argument is used to check type of generic T and returns correct return type for function.

Additional information about the issue

No response

[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.