nMigen sign extension issue

foo.py

from nmigen import *
from nmigen_boards.icebreaker import ICEBreakerPlatform
from nmigen.cli import main

class Foo(Elaboratable):

    def __init__(self):
        self.bar = Signal(signed(24))

    def elaborate(self, platform):
        led0 = platform.request('user_led', 0)
        led1 = platform.request('user_led', 1)
        m = Module()
        m.d.sync += self.bar.eq(self.bar - 1)
        m.d.comb += led0.eq(self.bar[-1])
        m.d.comb += led1.eq(self.bar[-2])
        return m


if __name__ == '__main__':
    platform = ICEBreakerPlatform()
    design = Foo()
    platform.build(design, do_program=True)

The LEDs show that the counter is incrementing, not decrementing.

Created nMigen issue #271.