type assertion

[simple vessel]

i have a union type t3 where only one of the fields can be defined

type t1 = {
  a: int;
  b: undefined;
}

type t2 = {
  a: undefined;
  b: int;
}

type t3 = t1 | t2;

v1 and v2 are int | undefined but i know that only one of the values will be set. how can i assert that a is t3?

const { v1, v2 } = values

const a: t3 = {
  a: v1,
  b: v2,
}

[full oxide]

once you destructure, v1 and v2 no longer have the "only one is set" linkage as t3 does.

same for once you property access actually

you could spread values in if the keys were the same

[simple vessel]

keys aren't the same so is type assertion the only way? are type assertions an escape hatch from TS