`Function.bind` in TypeScript with typing

function-bind.ts

export interface Fn<T, P extends any[], R> {
	(this: T, ...args: P) : R;
}

export type UnboundParams<F extends Fn<any, [ ...BindP, ...any[] ], any>, BindP extends any[]>
	= F extends Fn<any, [ ...BindP, ...infer CallP ], any> ? CallP : never;

export type BoundFn<F extends Fn<any, [ ...BindP, ...any[] ], any>, BindP extends any[]>
	= F extends Fn<infer T, [ ...BindP, ...infer CallP ], infer R>
		? Fn<T, CallP, R>
		: never;

export function bind<
	T,
	R,
	BindP extends any[],
	F extends Fn<T, [ ...BindP, ...UnboundParams<F, BindP> ], R>,
>(fn: F, this_val: T, ...params: BindP) : BoundFn<F, BindP> {
	return fn.bind(this_val, ...params);
}