Generic type resolving broken in typescript@next

[awerlogus]

Bug Report

🔎 Search Terms generic type resolve

🕗 Version & Regression Information 4.4.0-dev.20210605

💻 Code

/** @template T @typedef {T | undefined} Option */

/** @type {<T1, T2>(x: Option<T1>, y: T2) => T1 | T2} */
const a = (x, y) => x !== undefined ? x : y

// Making 'number | undefined' value
/** @type {{ prop?: number }} */
const b = { prop: 1 }
const { prop } = b

// 4.3: number
// 4.4: number | undefined
const c = a(prop, 2)

🙁 Actual behavior

undefined is a part of generic T1

🙂 Expected behavior

undefined is excluded from generic T1

[RyanCavanaugh]

I triaged a duplicate of this yesterday and can't find it, so tagging this as bug 😬. It repros in non-JS Doc type space as well

[ahejlsberg]

@awerlogus I'm unable to repro this issue. Can you let us know exactly what your config settings are?

[ahejlsberg]

@awerlogus Actually, it does repro, just not in the playground for some reason. It is fixed by #44565 and basically a duplicate of #44388, so I will close it as such.