constructor.ts

const enum Test { a = 'a' }
type Protocol = {
    foo: { x: string };
    bar: { y: number };
}
type WithTag<T> = {
    [TKey in keyof T]: { type: TKey } & T[TKey]
}
type ToDiscriminatedUnion<P> = WithTag<P>[keyof P]
type Constructor<T> = {
    [P in keyof T]: (value: T[P]) => { type: P } & T[P]
}
const Constructor: <P>() => Constructor<P> = <P>() =>
    new Proxy<Constructor<P>>({} as any, {
        get: (target, type) => (args) =>
            ({...args, type})
    })
type Constructor2<T> = {
    [P in keyof T]: (value: T[P]) => ToDiscriminatedUnion<T>
}
const Constructor2: <P>() => Constructor2<P> = <P>() =>
    new Proxy<Constructor2<P>>({} as any, {
        get: (target, type) => (args) =>
            ({...args, type})
    })
const Protocol = Constructor2<Protocol>()
const res = Protocol.foo({ x: 'x' })
alert(JSON.stringify(res))

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