When using signal with [HasEmitter] attribute from external module, c compiler report: implicit declaration of function ... [-Wimplicit-function-declaration]

for example if we have a module (test.vala) with class declaration:

public class Test : Object {
	[HasEmitter]
	public signal void test ();
}

and we use it in another module (test-main.vala):

static int main (string[] args) {
	Test test = new Test();
	test.test();
	return 0;
}

> valac test.vala test-main.vala report:
test-main.vala.c: In function ‘_vala_main’:
test-main.vala.c:36:2: warning: implicit declaration of function ‘test_test’ [-Wimplicit-function-declaration]