dmd linker (OPTLINK) gives Error 42: Symbol Undefined when using extern

[Andrej Mitrovic]

You can't prototype D functions like that, they belong to a module.
test is built as _D1b4testFZAya, not _D1a4testFZAya (notice the module
names, a and b after _D1).

What exactly are you trying to do? DLLs?