With the following non-member method declaration:

  void foo (...) {
    var l = va_list();
  }

valac does not consider it an error that the method has no named arguments, but the generated code does not work:

  #include <stdarg.h>
  
  void foo (...) {
    va_list l;
    va_start(l);
    va_end(l);
  }

The C code cannot be compiled because va_start is missing its required first argument. There is no way to access foo's arguments unless vala inserts a pretend argument in the C code, e.g.:

  void foo (void *marker, ...) {
    va_list l;
    va_start(l, marker);
    va_end(l);
  }

vala could automatically insert the argument into invocations of foo(), but people using vala functions from C would have to provide that argument themselves, which is ugly. The other option appears to be refusing to allow a variadic method to be declared without any named arguments.

A member method always has the instance passed as the first argument, so foo could in principle work as a member method. valac currently doesn't generate appropriate code for that:

  void test_foo (Test *self, ...) {
    va_list l;
    va_start(l);
    va_end(l);
  }

Presumably it would be a simple fix to make va_list work in this case.

It might be desirable not to fix it, though, for consistency with not allowing variadic non-member methods without named arguments.