enum TestEnum {
    A, B, C
}

void f(Gee.List<TestEnum> list) {
    list.foreach ((j) => {
        j = list.get(0);
        return true;
    });
}

Produces following code:

static gboolean __lambda2_ (Block1Data* _data1_, TestEnum j) {
	// ....
}


static gboolean ___lambda2__gee_forall_func (gpointer g, gpointer self) {
	gboolean result;
	result = __lambda2_ (self, g); // <- cast from gpointer to TestEnum
	return result;
}