If a generic class (with type parameter T) has a property of type T, the generated C code will use g_value_[get|set]_pointer() for the property marshalling. However, it will pass the value from the property’s getter method (which has return type gconstpointer) into g_value_set_pointer(), which takes a gpointer. This means GCC will complain about discarding the const qualifier from the pointer because Vala is violating the C type system.

For example:

public abstract class Folks.AbstractFieldDetails<T> : Object
{
  public virtual T @value
    {
      get { return this._value; }
      construct set { this._value = value; }
    }
}

generates the following:

static void _vala_folks_abstract_field_details_get_property (GObject * object, guint property_id, GValue * value, GParamSpec * pspec) {
	…
	switch (property_id) {
		case FOLKS_ABSTRACT_FIELD_DETAILS_VALUE:
		g_value_set_pointer (value, folks_abstract_field_details_get_value (self));
		break;
		…
	}
}

gconstpointer folks_abstract_field_details_get_value (FolksAbstractFieldDetails* self) {
	g_return_val_if_fail (self != NULL, NULL);
	return FOLKS_ABSTRACT_FIELD_DETAILS_GET_CLASS (self)->get_value (self);
}


This is with Vala 0.22.0.26-f79fe.