Generics in union should be inferred as `never` when possible

🔎 Search Terms

generics, inference, union, never

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about union and never.

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYewdgzgLgBAZjAvDAPAFQDQD4AUAPALhjRgB8ZoAnASzAHMMYBPIk8sAVwFsAjAU0oBKJFmYAoODgDkAQymMAjMIgyo1CHGp8IMTrwEwA9IeIx1MAEQyLukAHcxY0JFh0kqEnzxQ+YACY6HP58mmB8friEpuzc-EIiMHhidDhKFKrqmtq6sQbGpuZBfiG04bZ2QA

💻 Code

const f = <T,>(x: T | string, y: T | number) => y
f('a', 1) satisfies number // T is "a" now

const g = <T extends undefined>(x: T | number) => x
g(1) satisfies number // T is undefined now

🙁 Actual behavior

T is not never.

🙂 Expected behavior

T should be inferred as never, because it can be anything.

Additional information about the issue

This may be related to #42491.

リンク先の TypeScript Playground の再現から始め、関連する issue #42491 とジェネリック推論の挙動を比較してください。両方の例で T が never として推論され、satisfies チェックが期待どおりの結果を生成すれば、作業は完了です。