Discriminating property with never as possible type makes its enclosing object type disappear when narrowing

🔎 Search Terms

discriminated union never, discriminating property never

🕗 Version & Regression Information

It happens in every version I tried.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.0-dev.20241206#code/C4TwDgpgBAIglgZwMYFUB2cD2aoF4BQAPlAN5QAmiSACgE6ZgBcUaEAbhLQDRQCGzARigBfIqQpU6DZgCIw9MDJ4AjZgCYRYspWRSmUAK5pyEAGZxW5HkmYBmTfhNIANr1rQk2BMCiZm8ZHQsNHx8OFMACkwAOh0aBQBKUnwoXxSoAHoMqAA9AH58YSgIZwRoEnTMdKzcgtEwyIBCGLi9JIrUqtSa-MLi0vLK6uze4SA

💻 Code

type DiscUnion =
| { discProp: never, a: 1 }
| { discProp: "prop", b: 2 }
| { discProp: undefined, c: 3 }

declare const o: DiscUnion

if(o.discProp) {
  o
  // ^? { discProp: "prop"; b: 2; }
} else {
  o
  // ^? { discProp: undefined; c: 3; }
}

if(!o.discProp) {
  o
  // ^? { discProp: undefined; c: 3; }
} else {
  o
  // ^? { discProp: "prop"; b: 2; }
}

🙁 Actual behavior

{ discProp: never, a: 1 } disappears!

🙂 Expected behavior

In the first if, I expect { discProp: never, a: 1 } to be a possible type of o in the else branch. Viceversa, in the second if I expect it to be a possible type of o in the then branch.

Additional information about the issue

No response

Inizia con la riproduzione fornita in TypeScript Playground ed esegui l'esempio DiscUnion così com'è scritto. Traccia il narrowing sia per if(o.discProp) sia per if(!o.discProp); il lavoro è completato quando il membro { discProp: never, a: 1 } rimane possibile nei rami previsti senza modificare gli altri membri ristretti.