patch 9.0.1959: Vim9: methods parameters and types are covariant

Problem:  Vim9: methods parameters and types are covariant
Solution: Support contra-variant type check for object method arguments
          (similar to Dart).

closes: #12965
closes: #13221

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>
diff --git a/src/proto/vim9class.pro b/src/proto/vim9class.pro
index 9edf354..31e2be7 100644
--- a/src/proto/vim9class.pro
+++ b/src/proto/vim9class.pro
@@ -29,5 +29,5 @@
 void method_not_found_msg(class_T *cl, vartype_T v_type, char_u *name, size_t len);
 void member_not_found_msg(class_T *cl, vartype_T v_type, char_u *name, size_t len);
 void f_instanceof(typval_T *argvars, typval_T *rettv);
-int class_instance_of(class_T *cl, class_T *other_cl);
+int class_instance_of(class_T *cl, class_T *other_cl, int covariance_check);
 /* vim: set ft=c : */