Typescript merge enums

Question

I'm trying to merge enum maps, the code runs:

enum One {
    a = 'a',
}

enum Two {
    aa = 'aa',
}

enum Three {
    aaa = 'aaa',
}

type unit = One | Two | Three;

const twoFromOne: Map<Two, One> = new Map([[Two.aa, One.a]]);
const threeFromTwo: Map<Three, Two> = new Map([[Three.aaa, Two.aa]]);
const combined: Map<unit, unit> = new Map([
    ...twoFromOne,
    ...threeFromTwo,
]);

but I get a typescript compiler error:

const twoFromOne: Map<Two, One>
No overload matches this call.
  Overload 1 of 3, '(iterable: Iterable<readonly [Two, One]>): Map<Two, One>', gave the following error.
    Argument of type '([Two, One] | [Three, Two])[]' is not assignable to parameter of type 'Iterable<readonly [Two, One]>'.
      The types returned by '[Symbol.iterator]().next(...)' are incompatible between these types.
...

I don't understand the error, is it saying one map is being assigned into another instead of a merge?

Link to TS playground

Answer 1

The problem is the following:

TypeScript somehow infers the type based on only the first map that you provide ( ...twoFromOnw). You can add <unit, unit> to the MapConstructor (right side), so TypeScript will not infers thats type, rater use Map<unit, unit>:

const combined: Map<unit, unit> = new Map<unit, unit>([
    ...threeFromTwo,
    ...twoFromOne,
]);