Warning CLF_MISSING_IMPLEMENTATION_EXT issued on external class extending a d.ts class

[mmews-n4]

Example below:

Module.n4js

import {TSClass} from "TS";

export external public class TSClass2 extends TSClass {
    // Warning showing at    ^^^^^^^^
    // "External class TSClass2 must either be declared abstract or explicit declare method Clash.iClashMethod."
}

TS.d.ts

export class TSClass extends Clash {
    tsMethod(): string;
}

export class Clash {}

export interface Clash {
    iClashMethod(): number;
}