Discriminated union not narrowing when discriminator is itself a union

[Peeja]

Bug Report

🔎 Search Terms

discriminated union, discriminator, narrowing

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about discriminated unions

⏯ Playground Link

Playground link with relevant code

💻 Code

type DiscriminatedUnion = A | B | CorD;

interface A {
  type: "a";
}

interface B {
  type: "b";
}

interface CorD {
  type: "c" | "d";
}

declare const thing: DiscriminatedUnion;

thing.type;
// ^? const thing: DiscriminatedUnion
thing.type;
//    ^? (property) type: "a" | "b" | "c" | "d"

if (thing.type === "a") throw "No good!";

thing.type;
// ^? const thing: B | CorD
thing.type;
//    ^? (property) type: "b" | "c" | "d"

if (thing.type === "c" || thing.type === "d") throw "No good!";

thing.type;
// ^? const thing: B | CorD
thing.type;
//    ^? (property) type: "b"

🙁 Actual behavior

thing.type at the end has type B | CorD.

🙂 Expected behavior

thing.type at the end should have type B, because it can never be CorD.

---

This may well be considered a feature that hasn't been implemented yet.

For context, here's the type I'm trying to remove. This is an AST for SQL expressions. I've parsed an expression using this library, which gives me an Expr, but I'm only expecting to handle simpler kinds of SQL expressions, and I'd like to throw on more complex ones and narrow the type. Currently, it's harder than I'd like to kick SelectFromUnion out of the large Expr union, because its type is 'union' | 'union all'. Probably my best bet at this point would be a custom type guard, but it sure would be nice if TS could figure out the narrowing for me.

[MartinJohns]

Duplicate of #31404.

[Peeja]

@MartinJohns Thanks!

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[microsoft-github-policy-service]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[microsoft-github-policy-service]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.