Builder's join method to accept Builder as $second parameter

[res0]

I know we already have Subquery Joins, but there's no way (except raw query) to use subquery in the $second parameter for joins

Currently we can achieve it like this:

$builder = DB::table('authors as a')
  ->select('a.name', 'next_book.name as nextBookName')
  ->join(
    'books as next_book',
    'next_book.id',
    '=',
    DB::raw('(SELECT id FROM books WHERE author_id = a.id AND released_at > NOW() ORDER BY released_at ASC LIMIT 1)')
  );

But it would be nice to use it like this:

$subQuery = DB::table('books')
  ->where('author_id', DB::raw('a.id'))
  ->where('released_at', '>', DB::raw('NOW()'))
  ->orderBy('released_at')
  ->limit(1);

$builder = DB::table('authors as a')
  ->select('a.name', 'next_book.name as nextBookName')
  ->join('books as next_book', 'next_book.id', '=', $subQuery);

What do you think?