Type guards and discriminated unions: discriminant loss with optional properties

Answer

The discriminant is discarded when a type guard checks an optional discriminant property (e.g., x.kind === 'A' where kind may be undefined). Because the property might be missing, TypeScript’s control‑flow analysis cannot guarantee the tag exists, so it treats the checked value as if the discriminant were unknown and the narrowed type falls back to the original union or to never.

Confirmed facts

• If the discriminant is required (non‑optional) in every union member, the same guard preserves the tag and correctly narrows the union to the matching member.
• Making the discriminant non‑optional eliminates the narrowing collapse.
• When multiple consecutive guards each rely on an optional discriminant, after the first guard the discriminant is considered lost; subsequent guards therefore operate on never or the full union and provide no further narrowing.

Steps to verify this behavior

1. Create a TypeScript file with a discriminated union where the discriminant is optional, e.g.:

   type A = { kind: 'A'; a: number } | { kind: 'B'; b: string };

2. Write a type guard that checks the optional discriminant:

   function isA(x: A): x is Extract<A, { kind: 'A' }> {
     return x.kind === 'A';
   }

3. Inside the guarded block, hover over x or trigger a type error; you will see the type is the full union A (or never if the check is contradictory).
4. Repeat with the discriminant made required (remove ?) and observe that the type narrows to { kind: 'A'; a: number }.
5. Chain two guards and note that the second guard does not further narrow the type when the discriminant is optional.

No additional diagnostic detail is needed to change this recommendation.