You're really supposed to provide an explicit port list iirc.

from nmigen import *
from nmigen.back import verilog

class Test(Elaboratable):
    def __init__(self):
        self.val_out = Signal()

    def elaborate(self, platform):
        m = Module()
        int_sig = Signal(8)
        m.d.comb += self.val_out.eq(int_sig)

        return m

if __name__ == "__main__":
    top = Test()
    with open("test.v", "w") as f:
        f.write(verilog.convert(top, ports=[top.val_out]))

/* Generated by Yosys 0.9+3755 (git sha1 442d19f6, clang 12.0.0 -fPIC -Os) */

(* \nmigen.hierarchy  = "top" *)
(* top =  1  *)
(* generator = "nMigen" *)
module top(val_out);
  wire [7:0] int_sig;
  output val_out;
  assign int_sig = 8'h00;
  assign val_out = 1'h0;
endmodule

Thanks @BracketMaster, that does fix the issue at the top level.

I think there are a couple of usability issues here:

• If inferring ports in Verilog generation doesn't work then ports= shouldn't be an optional argument.
• The actual issue was an undriven signal many layers deep in the hierarchy. It was confusing to see this signal in a bunch of internal module ports when looking at waveforms. I presume every undriven signal makes it's way up to the top level.

Let's deprecate optional ports= in 0.3 and remove it in 0.4.