Type guards and discriminated unions: discriminant loss with optional properties

When a type guard predicated on a discriminated union includes an optional discriminant, the control‑flow analysis may discard the discriminant tag, causing the narrowed type to collapse to `never` or revert to the full union without an apparent trigger. This behavior interferes with reliable type narrowing in libraries that model optional state through discriminated unions. The precise condition under which the discriminant is lost remains unspecified in the type system documentation, and compiler output does not flag the discrepancy.

Developers encountering this pattern report that adding an explicit type assertion restores the expected narrowed type, but the underlying reason for the guard’s failure is not documented. The interaction between optional properties, the type predicate’s return type, and the discriminant tag appears to be the contributing factor, yet no definitive rule guides expected behavior.

Which conditions cause the discriminant to be discarded during type guard evaluation? Does making the discriminant non‑optional eliminate the narrowing collapse? How do multiple consecutive type predicates compound the effect on the discriminant?