U.Merge adds unexpected index signatures when applied to intersection types

[azhiv]

🐞 Bug Report

Describe the bug

U.Merge adds unexpected index signatures when applied to intersection types of the same content.

Reproduce the bug

import { Union } from "ts-toolbelt";

type discriminatedUnion = { flag: true } | { flag: false };
type simpleMerge = Union.Merge<discriminatedUnion>;

/* type simpleMerge = { flag: boolean; } */

type complexDiscriminatedUnion = ({ flag: true } | { flag: false }) & ({ flag: true } | { flag: false });
type complexMerge = Union.Merge<complexDiscriminatedUnion>;

/* type complexMerge = {
    [x: string]: undefined;
    [x: number]: undefined;
    [x: symbol]: undefined;
    flag: boolean;
} */

Playground link

Expected behavior

No index signatures are observed in complexMerge.

Possible Solution

Screenshots

Additional context

ts-toolbelt v.9.6.0