Strange behaviour of rdmd vs. dmd concerning main function

[berni via Digitalmars-d-learn]

I've got two source files in two directories:

Common/common.d
module common;

import std.stdio;

int main(string[] args)
{
   Foo foo = cast(Foo)Object.factory("special.Bar");

   foo.do_something();

   return 0;
}

abstract class Foo {
   abstract void do_something();
}

Special/special.d

module special;

import std.stdio;
import common;

class Bar : Foo {
   override void do_something()
   {
       writeln("works");
   }
}

Now I try to run it with rdmd and dmd and get quite different 
results:

$> rdmd -ICommon Special/special.d
works
$> dmd -ICommon Special/special.d
/usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/Scrt1.o: In function 
`_start':
(.text+0x20): undefined reference to `main'
special.o:(.data.rel.ro+0x18): undefined reference to 
`_D6common12__ModuleInfoZ'
special.o:(.data._D7special3Bar7__ClassZ+0x50): undefined 
reference to `_D6common3Foo7__ClassZ'
collect2: error: ld returned 1 exit status
Error: linker exited with status 1

I encountered this, when I was curious if I can move the main 
function to a diffent module, because it will be the same in 
several modules anyway. When testing with rdmd, everything 
worked. Later, I found out, that with dmd it doesn't. (And with 
gdc and ldc2 it doesn't work too.)

For me, this looks like a bug in either rdmd or dmd. But maybe 
there is something about it that I do not understand.