"undefined reference to" error. can't compile with custom modules

[Andrey]

On Sunday, 16 September 2012 at 14:13:02 UTC, Adam D.

Well, sometimes it works and sometimes doesn't. For example, if I 
exclude std.stdio import from the module file, then it works just 
with "dmd tests.d". Although yersterday it worked fine even with 
imported stdio. It this undefined behavior? What it depends on?