TypeScript typings error for handlers

[jakehamilton]

The current types in index.d.ts produce an error when attempting to use them in a purely TypeScript project:

Exported variable 'args' has or is using name 'flagSymbol' from external module
"/path/to/project/node_modules/arg/index" but cannot be named.ts(4023)

After a brief bit of tweaking, I found that this can be fixed by moving flagSymbol into the arg namespace and exporting it along with other members. This makes the type accessible and resolves the error. I'm unsure if there is a better way to handle it, though this should be fine.

My Local arg.d.ts file

declare module "arg" {
	function arg<T extends arg.Spec>(
		spec: T,
		options?: arg.Options
	): arg.Result<T>;

	namespace arg {
		export const flagSymbol: unique symbol;

		export function flag<T>(fn: T): T & { [arg.flagSymbol]: true };

		export const COUNT: Handler<number> & { [arg.flagSymbol]: true };

		export type Handler<T = any> = (
			value: string,
			name: string,
			previousValue?: T
		) => T;

		export class ArgError extends Error {
			constructor(message: string, code: string);

			code: string;
		}

		export interface Spec {
			[key: string]: string | Handler | [Handler];
		}

		export type Result<T extends Spec> = { _: string[] } & {
			[K in keyof T]?: T[K] extends Handler
				? ReturnType<T[K]>
				: T[K] extends [Handler]
				? Array<ReturnType<T[K][0]>>
				: never;
		};

		export interface Options {
			argv?: string[];
			permissive?: boolean;
			stopAtPositional?: boolean;
		}
	}

	export = arg;
}

---

Note: This is occurring for me using typescript@^4.6.3. I'm not sure if this is an issue with a new version of TypeScript or has always been a problem since I've typically only used arg in JavaScript projects up until now.