Yup, it's a bug and I think I know where.

I don't know if it helps, but I reduced it a bit further:

from nmigen import *
from nmigen.cli import main

m = Module()

a = Signal(1)
array = Array(x for x in range(2))

m.d.sync += a.eq(array[a])
        
if __name__ == "__main__":
    main(m, ports=[a])

fails similarly (https://hastebin.com/qujiciyihe.sql).

Oh, I figured it out, thanks! It's an issue with how legalizer expands arrays into switches; it does that by replacing the array index with a constant. But, in this case, you also have the array index on left-hand side, which shouldn't be replaced with a constant.

Ah, I understand. So part also suffers from the same problem:

from nmigen import *
from nmigen.cli import main

m = Module()

p = Signal(5, reset=0b11)
a = Signal(2, reset = 1)

m.d.sync += Cat(p.part(a, 1), a).eq(a)

if __name__ == "__main__":
    main(m)

Actually looking at the Part handling I think I noticed another bug: #52.

Thanks for the report! By the way, you can use Array(range(2)) just as well, you don't need the x for x in ... part.