Didn't find an existing issue for this. Sorry, if missed.

As described here: 
https://forum.dlang.org/post/iizchhylkxistlnbijzd@forum.dlang.org

There is an error during an attempt to fill an array of Nullable!uint during the execution of

void main()
{
    import std.typecons;
    Nullable!uint[] arr;
    arr.length = 5;
    arr[] = 1;
}

The suggested workaround is, currently, using a cast:
arr[] = cast(Nullable!uint)1;

Also an option would be: 
import std.algorithm;
fill(arr, 1);