The bitfields setters give incorrect results if the bitfield is 64 bits long
and the first field is <= 32.

The following program returns the correct result:

import std.bitmanip;
import std.string;

public static void main() {
    union  S {
        // entire 64-bit unsigned integer
        ulong bits = ulong.max;
        // split in two
        mixin (bitfields!(
            ulong, "back",  31,
            ulong, "front", 33)
        );

        const string toHex() { return format("0x%016X", bits); }
    }

    S num;

    num.bits = ulong.max;
    writeln("num1 = ", num.toHex);
    num.back = 1;
    writeln("num2 = ", num.toHex);
}

Output:
num1 = 0xFFFFFFFFFFFFFFFF
num2 = 0xFFFFFFFE00000001

But if you change the bitfields to this:

 mixin (bitfields!(
     ulong, "back",  32,
     ulong, "front", 32)
 );

Output:
num1 = 0xFFFFFFFFFFFFFFFF
num2 = 0x0000000000000001

The front half, which shouldn't be changed, is converted to zeros.

Or:

 mixin (bitfields!(
     ulong, "back",  31,
     ulong, "front", 33)
 );

Output:
num1 = 0xFFFFFFFFFFFFFFFF
num2 = 0x0000000080000001

I experimented a little bit.

1) The sizes of the fields (ubyte, ushort, etc.) don't seem to matter as long as they are long enough to hold the value.

2) It's only the first field that is broken, as far as I can tell.

3) Additional fields don't solve the problem.

I looked at the code for std.bitmanip but it's beyond my ability to modify. So I'll have to rely on the kindness of strangers for a fix.