#  import std.stdarg;
#  import std.stdio;
#  int main() {
#     void *v = null;
#     va_arg!(int)(v);
#     writef("\n");
#     return 0;
#  }

Linker complains:
phobos.lib(format)  Offset 13726H Record Type 00C3
Error 1: Previous Definition Different : _D3std6stdarg8va_arg_i6va_argFKPvZ
--- errorlevel 1

If I either remove va_arg or writef everything compiles/links just fine.