I was getting quite obscure linker errors in a project. I reduced it to this:
__FILE__ generates sometimes relative paths even though all files and '-I' paths are given as absolute paths, depending on the directory in which 'dmd' is launched.



----------------------------
/mydir/fun1.d:
import fun2;
void test(){ test2(); }
/mydir/fun2.d:
void test2(string file=__FILE__)(){
  if(false) test2();
}

-----------------
cd /mydir/
dmd -of/tmp/fun1.o -c -I/mydir/ /mydir/fun1.d && nm /tmp/fun1.o |ddemangle |grep test2
S pure nothrow @nogc @safe void fun2.test2!("/mydir/fun1.d").test2()
S pure nothrow @nogc @safe void fun2.test2!("fun2.d").test2()
-----------------
cd / #or anywhere else

dmd -of/tmp/fun1.o -c -I/mydir/ /mydir/fun1.d && nm /tmp/fun1.o |ddemangle |grep test2
S pure nothrow @nogc @safe void fun2.test2!("/mydir/fun1.d").test2()
S pure nothrow @nogc @safe void fun2.test2!("/mydir/fun1.d").test2()

The difference is: 'test2!("/mydir/fun1.d")' vs 'test2!("fun2.d")'

I believe 'test2!("fun2.d")' is wrong.

Maybe related:
http://www.digitalmars.com/d/archives/digitalmars/D/learn/exclude_current_directory_from_search_path_in_dmd_69919.html "exclude current directory from search path in dmd ?" (which i asked on the forum but didn't get answer to)