Generics are inferred wrong in a union

[awerlogus]

🔎 Search Terms

generic union inference

🕗 Version & Regression Information

• This is a crash
• This changed between versions ______ and _______
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.3.0-dev.20231031#code/CYUwxgNghgTiAEYD2A7AzgF3lAXPAPAKIA08AKgHwAUIAlhgBYgx4DaAZlBGiKYQLrwAPvFYYYAV17l+ASngBeCvABuSWsACwAKB2hIsBMnRYARm07dpmGLRQBzQSLGTpKCQFtTzfjp1QqU1kdIA

💻 Code

declare const a: <E, T>(either: [false, E] | [true, T]) => void

declare const b: [false, string] | [true, number]

// now:      const a: <string, string>(either: [false, string] | [true, string]) => void
// expected: const a: <string, number>(either: [false, string] | [true, number]) => void
a(b)

🙁 Actual behavior

Both parameters: E and T are inferred as strings

🙂 Expected behavior

Parameter E is inferred as a string, parameter T is inferred as a number

Additional information about the issue

No response

[jcalz]

Possibly related to #45809?

Also looks like #55648 (comment)

[RyanCavanaugh]

Agree this is duplicate of #45809; tuple and object type discriminants work the same way