Bug in conditional types with union types: AProperty should be number instead of never

[jerryyxu]

🔎 Search Terms

infer T in conditional type union returns never

🕗 Version & Regression Information

I started encountering this issue in TypeScript v5.7.3

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.7.3#code/C4TwDgpgBAglC8UDeUBEBDVAuKA7ArgLYBGEATlAL5QA+U+uAJhAGYCWuEjA3AFC+hIsAApkA9pDKgEsKBAAewCEwDOyNJhwcW5KABUqUAPz6oOTgDdyfXgGMxuFcCjocMURPLTEARiA

💻 Code

type A = { "a": number } | undefined;

type AProperty = A extends { "a": infer T } ? T : never;

const a: AProperty = 1

🙁 Actual behavior

TypeScript is returning AProperty as never.

🙂 Expected behavior

I expected AProperty to resolve to number, as the type { "a": number } exists within the union type A.

Additional information about the issue

No response

[MartinJohns]

This is working correctly. The type { a: number } | undefined does not extend the type { a: number }.

Note: Unless the type you check (A in this case) is a generic type you will not get distributive behavior. Perhaps you expected this?