3.5 Unions and optionals

A | B — an anonymous union. Unions are flat (A | (B | C) = A | B | C), the order does not matter, duplicates are removed.
Subtyping: A is a subtype of A | B; a value of a subtype is implicitly converted to the union.
T? is shorthand for T | none.
Type narrowing: x is T in a condition, match, ?, ??, ?., ! (§5, §6.5). After if x is T the variable x has type T in the branch if it is immutable or was not modified in the branch.
A union that contains at least one type with @error (§6.5) is a result with an error; it cannot be ignored.

Related diagnostics