The warning does not occur with dmd or 32-bit gdc.
wconv.d:
import std.string;
void main()
{
auto s = "hello world";
auto pos = s.find(' ');
auto hello = s[0 .. pos];
}
$ gdmd -w wconv
warning - wconv.d:7: Error: implicit conversion of expression (pos) of type long to ulong can cause loss of data