if I try dmd main.d helpers.d it works
but if you run build.bat it fails

main.obj(main)
 Error 42: Symbol Undefined
_Dhelpers__L8Interface__0_cast_to_FPS8com_GUIDC6Obje
ctPPvZi

build.bat is ....
dmd -c helpers.d
lib mylib +-helpers.obj,,
dmd -c main.d
set LIB=c:\dmd\lib;c:\dm\lib
link main.obj,main,,mylib+phobos+snn+uuid/noi

I rely on this for my build env, as I have over 32 d files to include and I
started getting complains from dmd about cmd line length.
(and it allows me to share code between projects).