dynamic choice options

We have dynamic default values via lambda, I'd see myself often hacking around what could be achieved with dynamic types, ideally with the context available to react upon already parsed options/arguments:

@click.group()
@click.option('--foo')
def cmd(foo):
    pass

@cmd.command()
@click.option('--bar')
@click.option('--tag', type=lambda ctx: click.Choice(dynamic_list_depending_on_foo_and_bar))

With the above I see the issue often knowing how many arguments tag is going to eat from the command line. Maybe better:

@click.group()
@click.option('--foo')
def cmd(foo):
    pass

@cmd.command()
@click.option('--bar')
@click.option('--tag', type=click.Choice(lambda ctx: dynamic_list_depending_on_foo_and_bar))

Start with the dynamic-defaults-for-prompts documentation and the issue's @click.group, @click.option, and click.Choice examples. Trace how parsed options and arguments are made available to a dynamic type, then define behavior for a choice list depending on foo and bar, including how many command-line arguments --tag consumes.