Dealing with Tagged Structs in TypeScript

interface A extends Data.Case {
    _tag: "A"
    a: number
}
const A = Data.tagged<A>(`A`)
interface B extends Data.Case {
    _tag: "B"
    b: string
}
const B = Data.tagged<B>(`B`)

declare const mylist: (A | B)[]

declare const wanted: {
   a: number
   b: string
}