patch 9.0.1906: Vim9: Interfaces should not support class methods and variables

Problem:  Vim9: Interfaces should not support class methods and
          variables
Solution: Make sure interface follow the interface specification

Vim9 interface changes to follow the new interface specification:

1) An interface can have only read-only and read-write instance
   variables.
2) An interface can have only public instance methods.
3) An interface cannot have class variables and class methods.
4) An interface cannot have private instance variables and private
   instance methods.
5) A interface can extend another interface using "extends". The
   sub-interface gets all the variables and methods in the super
   interface.

That means:
- Interfaces should not support class methods and variables.
- Adjust error numbers and add additional tests.
- Interface methods can be defined in one of the super classes.
- Interface variables can be defined in one of the super classes.
  and instance variables can be repeated in sub interfaces.
- Check the class variable types with the type in interface.

closes: #13100

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

src/errors.h

@@ -1572,8 +1572,7 @@ EXTERN char e_unknown_printer_font_str[]
 #endif
 EXTERN char e_class_required[]
 	INIT(= N_("E614: Class required"));
-EXTERN char e_object_required[]
-	INIT(= N_("E615: Object required"));
+// E615 unused
 EXTERN char e_object_required_for_argument_nr[]
 	INIT(= N_("E616: Object required for argument %d"));
 #ifdef FEAT_GUI_GTK
@@ -3401,8 +3400,7 @@ EXTERN char e_object_required_found_str[]
 	INIT(= N_("E1327: Object required, found %s"));
 EXTERN char e_constructor_default_value_must_be_vnone_str[]
 	INIT(= N_("E1328: Constructor default value must be v:none: %s"));
-EXTERN char e_cannot_get_object_member_type_from_initializer_str[]
-	INIT(= N_("E1329: Cannot get object member type from initializer: %s"));
+// E1329 unused
 EXTERN char e_invalid_type_for_object_member_str[]
 	INIT(= N_("E1330: Invalid type for object member: %s"));
 EXTERN char e_public_must_be_followed_by_this_or_static[]
@@ -3411,6 +3409,7 @@ EXTERN char e_public_member_name_cannot_start_with_underscore_str[]
 	INIT(= N_("E1332: Public member name cannot start with underscore: %s"));
 EXTERN char e_cannot_access_private_member_str[]
 	INIT(= N_("E1333: Cannot access private member: %s"));
+// E1334 unused
 EXTERN char e_member_is_not_writable_str[]
 	INIT(= N_("E1335: Member is not writable: %s"));
 #endif
@@ -3419,8 +3418,8 @@ EXTERN char e_internal_error_shortmess_too_long[]
 #ifdef FEAT_EVAL
 EXTERN char e_class_member_str_not_found_in_class_str[]
 	INIT(= N_("E1337: Class member \"%s\" not found in class \"%s\""));
-EXTERN char e_member_not_found_on_class_str_str[]
-	INIT(= N_("E1338: Member not found on class \"%s\": %s"));
+EXTERN char e_interface_static_direct_access_str[]
+	INIT(= N_("E1338: Cannot directly access interface \"%s\" static member \"%s\""));
 #endif
 #ifdef FEAT_PROP_POPUP
 EXTERN char e_cannot_add_textprop_with_text_after_using_textprop_with_negative_id[]
@@ -3444,9 +3443,9 @@ EXTERN char e_interface_name_not_found_str[]
 EXTERN char e_not_valid_interface_str[]
 	INIT(= N_("E1347: Not a valid interface: %s"));
 EXTERN char e_member_str_of_interface_str_not_implemented[]
-	INIT(= N_("E1348: Member \"%s\" of interface \"%s\" not implemented"));
-EXTERN char e_function_str_of_interface_str_not_implemented[]
-	INIT(= N_("E1349: Function \"%s\" of interface \"%s\" not implemented"));
+	INIT(= N_("E1348: Member \"%s\" of interface \"%s\" is not implemented"));
+EXTERN char e_method_str_of_interface_str_not_implemented[]
+	INIT(= N_("E1349: Method \"%s\" of interface \"%s\" is not implemented"));
 EXTERN char e_duplicate_implements[]
 	INIT(= N_("E1350: Duplicate \"implements\""));
 EXTERN char e_duplicate_interface_after_implements_str[]
@@ -3480,6 +3479,7 @@ EXTERN char e_incomplete_type[]
 #endif
 EXTERN char e_warning_pointer_block_corrupted[]
 	INIT(= N_("E1364: Warning: Pointer block corrupted"));
+#ifdef FEAT_EVAL
 EXTERN char e_cannot_use_a_return_type_with_new[]
 	INIT(= N_("E1365: Cannot use a return type with the \"new\" function"));
 EXTERN char e_cannot_access_private_method_str[]
@@ -3504,10 +3504,21 @@ EXTERN char e_class_member_str_accessible_only_using_class_str[]
 	INIT(= N_("E1375: Class member \"%s\" accessible only using class \"%s\""));
 EXTERN char e_object_member_str_accessible_only_using_object_str[]
 	INIT(= N_("E1376: Object member \"%s\" accessible only using class \"%s\" object"));
-EXTERN char e_static_member_not_supported_in_interface[]
-	INIT(= N_("E1377: Static member is not supported in an interface"));
 EXTERN char e_method_str_of_class_str_has_different_access[]
-	INIT(= N_("E1378: Access level of method \"%s\" is different in class \"%s\""));
+	INIT(= N_("E1377: Access level of method \"%s\" is different in class \"%s\""));
+EXTERN char e_static_cannot_be_used_in_interface[]
+	INIT(= N_("E1378: Static cannot be used in an interface"));
+EXTERN char e_private_variable_str_in_interface[]
+	INIT(= N_("E1379: Private variable not supported in an interface"));
+EXTERN char e_private_method_str_in_interface[]
+	INIT(= N_("E1380: Private method not supported in an interface"));
+EXTERN char e_interface_cannot_use_implements[]
+	INIT(= N_("E1381: Interface cannot use \"implements\""));
+EXTERN char e_member_str_type_mismatch_expected_str_but_got_str[]
+	INIT(= N_("E1382: Member \"%s\": type mismatch, expected %s but got %s"));
+EXTERN char e_method_str_type_mismatch_expected_str_but_got_str[]
+	INIT(= N_("E1383: Method \"%s\": type mismatch, expected %s but got %s"));
+#endif
 EXTERN char e_cannot_mix_positional_and_non_positional_str[]
 	INIT(= N_("E1400: Cannot mix positional and non-positional arguments: %s"));
 EXTERN char e_fmt_arg_nr_unused_str[]
@@ -3520,12 +3531,6 @@ EXTERN char e_positional_arg_num_type_inconsistent_str_str[]
 	INIT(= N_("E1404: Positional argument %d type used inconsistently: %s/%s"));
 EXTERN char e_invalid_format_specifier_str[]
 	INIT(= N_("E1405: Invalid format specifier: %s"));
-EXTERN char e_member_str_type_mismatch_expected_str_but_got_str[]
-	INIT(= N_("E1406: Member \"%s\": type mismatch, expected %s but got %s"));
-EXTERN char e_method_str_type_mismatch_expected_str_but_got_str[]
-	INIT(= N_("E1407: Method \"%s\": type mismatch, expected %s but got %s"));
 EXTERN char e_aptypes_is_null_nr_str[]
 	INIT(= "E1408: Internal error: ap_types or ap_types[idx] is NULL: %d: %s");
-EXTERN char e_interface_static_direct_access_str[]
-	INIT(= N_("E1409: Cannot directly access interface \"%s\" static member \"%s\""));
-// E1376 - E1399 unused
+// E1384 - E1399 unused

src/testdir/test_vim9_builtin.vim

@@ -2339,6 +2339,25 @@ def Test_instanceof()
     Bar()
   END
   v9.CheckScriptFailure(lines, 'E1013: Argument 2: type mismatch, expected class<Unknown> but got number')
+
+  lines =<< trim END
+    vim9script
+    class Foo
+    endclass
+    instanceof(Foo.new(), [{}])
+  END
+  v9.CheckSourceFailure(lines, 'E614: Class required')
+
+  lines =<< trim END
+    vim9script
+    class Foo
+    endclass
+    def Bar()
+      instanceof(Foo.new(), [{}])
+    enddef
+    Bar()
+  END
+  v9.CheckSourceFailure(lines, 'E614: Class required')
 enddef
 
 def Test_invert()

src/testdir/test_vim9_disassemble.vim

@@ -3052,9 +3052,7 @@ def Test_disassemble_interface_static_member()
   var lines =<< trim END
     vim9script
     interface I
-      public static s_var: number
       public this.o_var: number
-      public static s_var2: number
       public this.o_var2: number
     endinterface
 

src/version.c

@@ -699,6 +699,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1906,
 /**/
     1905,
 /**/

src/vim9class.c

@@ -293,7 +293,10 @@ object_index_from_itf_index(class_T *itf, int is_method, int idx, class_T *cl,
  * Returns TRUE if the class name "extends_names" is a valid class.
  */
     static int
-validate_extends_class(char_u *extends_name, class_T **extends_clp)
+validate_extends_class(
+    char_u  *extends_name,
+    class_T **extends_clp,
+    int	    is_class)
 {
     typval_T	tv;
     int		success = FALSE;
@@ -305,9 +308,13 @@ validate_extends_class(char_u *extends_name, class_T **extends_clp)
 	return success;
     }
 
-    if (tv.v_type != VAR_CLASS
-	    || tv.vval.v_class == NULL
-	    || (tv.vval.v_class->class_flags & CLASS_INTERFACE) != 0)
+    if (tv.v_type != VAR_CLASS || tv.vval.v_class == NULL
+	    || (is_class
+		&& (tv.vval.v_class->class_flags & CLASS_INTERFACE) != 0)
+	    || (!is_class
+		&& (tv.vval.v_class->class_flags & CLASS_INTERFACE) == 0))
+	// a interface cannot extend a class and a class cannot extend an
+	// interface.
 	semsg(_(e_cannot_extend_str), extends_name);
     else
     {
@@ -352,6 +359,8 @@ validate_extends_methods(
 	    if (extends_private)
 		pstr++;
 
+	    // When comparing the method names, ignore the access type (public
+	    // and private methods are considered the same).
 	    for (int j = 0; j < method_count; j++)
 	    {
 		char_u  *qstr = cl_fp[j]->uf_name;
@@ -380,12 +389,10 @@ validate_extends_methods(
  * are no duplicates.
  */
     static int
-validate_extends_members(
+extends_check_dup_members(
     garray_T	*objmembers_gap,
     class_T	*extends_cl)
 {
-    // loop == 1: check class members
-    // loop == 2: check object members
     int member_count = objmembers_gap->ga_len;
     if (member_count == 0)
 	return TRUE;
@@ -431,6 +438,68 @@ validate_extends_members(
     return TRUE;
 }
 
+/*
+ * Compare the variable type of interface variables in "objmembers_gap" against
+ * the variable in any of the extended super interface lineage.  Used to
+ * compare the variable types when extending interfaces.  Returns TRUE if the
+ * variable types are the same.
+ */
+    static int
+extends_check_intf_var_type(
+    garray_T	*objmembers_gap,
+    class_T	*extends_cl)
+{
+    int member_count = objmembers_gap->ga_len;
+    if (member_count == 0)
+	return TRUE;
+
+    ocmember_T *members = (ocmember_T *)(objmembers_gap->ga_data);
+
+    // Validate each member variable
+    for (int c_i = 0; c_i < member_count; c_i++)
+    {
+	class_T	    *p_cl = extends_cl;
+	ocmember_T  *c_m = members + c_i;
+	int	    var_found = FALSE;
+
+	// Check in all the parent classes in the lineage
+	while (p_cl != NULL && !var_found)
+	{
+	    int p_member_count = p_cl->class_obj_member_count;
+	    if (p_member_count == 0)
+	    {
+		p_cl = p_cl->class_extends;
+		continue;
+	    }
+	    ocmember_T *p_members = p_cl->class_obj_members;
+
+	    // Compare against all the members in the parent class
+	    for (int p_i = 0; p_i < p_member_count; p_i++)
+	    {
+		where_T		where = WHERE_INIT;
+		ocmember_T	*p_m = p_members + p_i;
+
+		if (STRCMP(p_m->ocm_name, c_m->ocm_name) != 0)
+		    continue;
+
+		// Ensure the type is matching.
+		where.wt_func_name = (char *)c_m->ocm_name;
+		where.wt_kind = WT_MEMBER;
+
+		if (check_type(p_m->ocm_type, c_m->ocm_type, TRUE,
+								where) == FAIL)
+		    return FALSE;
+
+		var_found = TRUE;
+	    }
+
+	    p_cl = p_cl->class_extends;
+	}
+    }
+
+    return TRUE;
+}
+
 /*
  * When extending an abstract class, check whether all the abstract methods in
  * the parent class are implemented.  Returns TRUE if all the methods are
@@ -491,60 +560,107 @@ validate_abstract_class_methods(
 }
 
 /*
- * Check the members of the interface class "ifcl" match the class members
- * ("classmembers_gap") and object members ("objmembers_gap") of a class.
- * Returns TRUE if the class and object member names are valid.
+ * Returns TRUE if the interface variable "if_var" is present in the list of
+ * variables in "cl_mt" or in the parent lineage of one of the extended classes
+ * in "extends_cl".  For a class variable, 'is_class_var' is TRUE.
  */
     static int
-validate_interface_members(
+intf_variable_present(
+    char_u	*intf_class_name,
+    ocmember_T *if_var,
+    int		is_class_var,
+    ocmember_T *cl_mt,
+    int		cl_member_count,
+    class_T	*extends_cl)
+{
+    int		variable_present  = FALSE;
+
+    for (int cl_i = 0; cl_i < cl_member_count; ++cl_i)
+    {
+	ocmember_T	*m = &cl_mt[cl_i];
+	where_T		where = WHERE_INIT;
+
+	if (STRCMP(if_var->ocm_name, m->ocm_name) != 0)
+	    continue;
+
+	// Ensure the access type is same
+	if (if_var->ocm_access != m->ocm_access)
+	{
+	    semsg(_(e_member_str_of_interface_str_has_different_access),
+		    if_var->ocm_name, intf_class_name);
+	    return FALSE;
+	}
+
+	// Ensure the type is matching.
+	if (m->ocm_type == &t_any)
+	{
+	    // variable type is not specified.  Use the variable type in the
+	    // interface.
+	    m->ocm_type = if_var->ocm_type;
+	}
+	else
+	{
+	    where.wt_func_name = (char *)m->ocm_name;
+	    where.wt_kind = WT_MEMBER;
+	    if (check_type(if_var->ocm_type, m->ocm_type, TRUE,
+							    where) == FAIL)
+		return FALSE;
+	}
+
+	variable_present = TRUE;
+	break;
+    }
+
+    if (!variable_present && extends_cl != NULL)
+    {
+	int ext_cl_count = is_class_var
+				? extends_cl->class_class_member_count
+				: extends_cl->class_obj_member_count;
+	ocmember_T *ext_cl_mt = is_class_var
+				? extends_cl->class_class_members
+				: extends_cl->class_obj_members;
+	return intf_variable_present(intf_class_name, if_var,
+					is_class_var, ext_cl_mt,
+					ext_cl_count,
+					extends_cl->class_extends);
+    }
+
+    return variable_present;
+}
+
+/*
+ * Check the variables of the interface class "ifcl" match the class variables
+ * ("classmembers_gap") and object variables ("objmembers_gap") of a class.
+ * Returns TRUE if the class and object variables names are valid.
+ */
+    static int
+validate_interface_variables(
     char_u	*intf_class_name,
     class_T	*ifcl,
     garray_T	*classmembers_gap,
-    garray_T	*objmembers_gap)
+    garray_T	*objmembers_gap,
+    class_T	*extends_cl)
 {
     for (int loop = 1; loop <= 2; ++loop)
     {
-	// loop == 1: check class members
-	// loop == 2: check object members
-	int if_count = loop == 1 ? ifcl->class_class_member_count
+	// loop == 1: check class variables
+	// loop == 2: check object variables
+	int is_class_var = (loop == 1);
+	int if_count = is_class_var ? ifcl->class_class_member_count
 						: ifcl->class_obj_member_count;
 	if (if_count == 0)
 	    continue;
-	ocmember_T *if_ms = loop == 1 ? ifcl->class_class_members
+	ocmember_T *if_ms = is_class_var ? ifcl->class_class_members
 						: ifcl->class_obj_members;
-	ocmember_T *cl_ms = (ocmember_T *)(loop == 1
+	ocmember_T *cl_ms = (ocmember_T *)(is_class_var
 						? classmembers_gap->ga_data
 						: objmembers_gap->ga_data);
-	int cl_count = loop == 1 ? classmembers_gap->ga_len
+	int cl_count = is_class_var ? classmembers_gap->ga_len
 						: objmembers_gap->ga_len;
 	for (int if_i = 0; if_i < if_count; ++if_i)
 	{
-	    int cl_i;
-	    for (cl_i = 0; cl_i < cl_count; ++cl_i)
-	    {
-		ocmember_T	*m = &cl_ms[cl_i];
-		where_T		where = WHERE_INIT;
-
-		if (STRCMP(if_ms[if_i].ocm_name, m->ocm_name) != 0)
-		    continue;
-
-		// Ensure the type is matching.
-		where.wt_func_name = (char *)m->ocm_name;
-		where.wt_kind = WT_MEMBER;
-		if (check_type(if_ms[if_i].ocm_type, m->ocm_type, TRUE,
-								where) == FAIL)
-		    return FALSE;
-
-		if (if_ms[if_i].ocm_access != m->ocm_access)
-		{
-		    semsg(_(e_member_str_of_interface_str_has_different_access),
-			    if_ms[if_i].ocm_name, intf_class_name);
-		    return FALSE;
-		}
-
-		break;
-	    }
-	    if (cl_i == cl_count)
+	    if (!intf_variable_present(intf_class_name, &if_ms[if_i],
+				is_class_var, cl_ms, cl_count, extends_cl))
 	    {
 		semsg(_(e_member_str_of_interface_str_not_implemented),
 			if_ms[if_i].ocm_name, intf_class_name);
@@ -557,56 +673,107 @@ validate_interface_members(
 }
 
 /*
- * Check the functions/methods of the interface class "ifcl" match the class
- * methods ("classfunctions_gap") and object functions ("objmemthods_gap") of a
- * class.
- * Returns TRUE if the class and object member names are valid.
+ * Returns TRUE if the method signature of "if_method" and "cl_method" matches.
+ */
+    static int
+intf_method_type_matches(ufunc_T *if_method, ufunc_T *cl_method)
+{
+    where_T where = WHERE_INIT;
+
+    // Ensure the type is matching.
+    where.wt_func_name = (char *)if_method->uf_name;
+    where.wt_kind = WT_METHOD;
+    if (check_type(if_method->uf_func_type, cl_method->uf_func_type, TRUE,
+								where) == FAIL)
+	return FALSE;
+
+    return TRUE;
+}
+
+/*
+ * Returns TRUE if the interface method "if_ufunc" is present in the list of
+ * methods in "cl_fp" or in the parent lineage of one of the extended classes
+ * in "extends_cl".  For a class method, 'is_class_method' is TRUE.
+ */
+    static int
+intf_method_present(
+    ufunc_T *if_ufunc,
+    int	    is_class_method,
+    ufunc_T **cl_fp,
+    int	    cl_count,
+    class_T *extends_cl)
+{
+    int		method_present  = FALSE;
+
+    for (int cl_i = 0; cl_i < cl_count; ++cl_i)
+    {
+	char_u *cl_name = cl_fp[cl_i]->uf_name;
+	if (STRCMP(if_ufunc->uf_name, cl_name) == 0)
+	{
+	    // Ensure the type is matching.
+	    if (!intf_method_type_matches(if_ufunc, cl_fp[cl_i]))
+		return FALSE;
+	    method_present = TRUE;
+	    break;
+	}
+    }
+
+    if (!method_present && extends_cl != NULL)
+    {
+	ufunc_T **ext_cl_fp = (ufunc_T **)(is_class_method
+					? extends_cl->class_class_functions
+					: extends_cl->class_obj_methods);
+	int	ext_cl_count = is_class_method
+				? extends_cl->class_class_function_count
+				: extends_cl->class_obj_method_count;
+	return intf_method_present(if_ufunc, is_class_method, ext_cl_fp,
+					ext_cl_count,
+					extends_cl->class_extends);
+    }
+
+    return method_present;
+}
+
+/*
+ * Validate that a new class implements all the class/instance methods in the
+ * interface "ifcl".  The new class methods are in "classfunctions_gap" and the
+ * new object methods are in "objmemthods_gap".  Also validates the method
+ * types.
+ * Returns TRUE if all the interface class/object methods are implemented in
+ * the new class.
  */
     static int
 validate_interface_methods(
     char_u	*intf_class_name,
     class_T	*ifcl,
     garray_T	*classfunctions_gap,
-    garray_T	*objmethods_gap)
+    garray_T	*objmethods_gap,
+    class_T	*extends_cl)
 {
     for (int loop = 1; loop <= 2; ++loop)
     {
-	// loop == 1: check class functions
+	// loop == 1: check class methods
 	// loop == 2: check object methods
-	int if_count = loop == 1 ? ifcl->class_class_function_count
+	int is_class_method = (loop == 1);
+	int if_count = is_class_method ? ifcl->class_class_function_count
 					: ifcl->class_obj_method_count;
 	if (if_count == 0)
 	    continue;
-	ufunc_T **if_fp = loop == 1 ? ifcl->class_class_functions
+	ufunc_T **if_fp = is_class_method ? ifcl->class_class_functions
 						: ifcl->class_obj_methods;
-	ufunc_T **cl_fp = (ufunc_T **)(loop == 1
+	ufunc_T **cl_fp = (ufunc_T **)(is_class_method
 						? classfunctions_gap->ga_data
 						: objmethods_gap->ga_data);
-	int cl_count = loop == 1 ? classfunctions_gap->ga_len
+	int cl_count = is_class_method ? classfunctions_gap->ga_len
 						: objmethods_gap->ga_len;
 	for (int if_i = 0; if_i < if_count; ++if_i)
 	{
 	    char_u	*if_name = if_fp[if_i]->uf_name;
-	    int		cl_i;
-	    for (cl_i = 0; cl_i < cl_count; ++cl_i)
-	    {
-		char_u *cl_name = cl_fp[cl_i]->uf_name;
-		if (STRCMP(if_name, cl_name) == 0)
-		{
-		    where_T where = WHERE_INIT;
 
-		    // Ensure the type is matching.
-		    where.wt_func_name = (char *)if_name;
-		    where.wt_kind = WT_METHOD;
-		    if (check_type(if_fp[if_i]->uf_func_type,
-			cl_fp[cl_i]->uf_func_type, TRUE, where) == FAIL)
-			return FALSE;
-		    break;
-		}
-	    }
-	    if (cl_i == cl_count)
+	    if (!intf_method_present(if_fp[if_i], is_class_method, cl_fp,
+							cl_count, extends_cl))
 	    {
-		semsg(_(e_function_str_of_interface_str_not_implemented),
+		semsg(_(e_method_str_of_interface_str_not_implemented),
 			if_name, intf_class_name);
 		return FALSE;
 	    }
@@ -630,7 +797,8 @@ validate_implements_classes(
     garray_T	*classfunctions_gap,
     garray_T	*classmembers_gap,
     garray_T	*objmethods_gap,
-    garray_T	*objmembers_gap)
+    garray_T	*objmembers_gap,
+    class_T	*extends_cl)
 {
     int		success = TRUE;
 
@@ -660,15 +828,16 @@ validate_implements_classes(
 	intf_classes[i] = ifcl;
 	++ifcl->class_refcount;
 
-	// check the members of the interface match the members of the class
-	success = validate_interface_members(impl, ifcl, classmembers_gap,
-							objmembers_gap);
+	// check the variables of the interface match the members of the class
+	success = validate_interface_variables(impl, ifcl, classmembers_gap,
+						objmembers_gap, extends_cl);
 
 	// check the functions/methods of the interface match the
 	// functions/methods of the class
 	if (success)
 	    success = validate_interface_methods(impl, ifcl,
-					classfunctions_gap, objmethods_gap);
+					classfunctions_gap, objmethods_gap,
+					extends_cl);
 	clear_tv(&tv);
     }
 
@@ -820,8 +989,7 @@ update_member_method_lookup_table(
     class_T	*ifcl,
     class_T	*cl,
     garray_T	*objmethods,
-    int		pobj_method_offset,
-    int		is_interface)
+    int		pobj_method_offset)
 {
     if (ifcl == NULL)
 	return OK;
@@ -876,7 +1044,7 @@ update_member_method_lookup_table(
 	// extended class object method is not overridden by the child class.
 	// Keep the method declared in one of the parent classes in the
 	// lineage.
-	if (!done && !is_interface)
+	if (!done)
 	{
 	    // If "ifcl" is not the immediate parent of "cl", then search in
 	    // the intermediate parent classes.
@@ -927,13 +1095,20 @@ update_member_method_lookup_table(
     static int
 add_lookup_tables(class_T *cl, class_T *extends_cl, garray_T *objmethods_gap)
 {
+    // update the lookup table for all the implemented interfaces
     for (int i = 0; i < cl->class_interface_count; ++i)
     {
 	class_T *ifcl = cl->class_interfaces_cl[i];
 
-	if (update_member_method_lookup_table(ifcl, cl, objmethods_gap,
-							0, TRUE) == FAIL)
-	    return FAIL;
+	// update the lookup table for this interface and all its super
+	// interfaces.
+	while (ifcl != NULL)
+	{
+	    if (update_member_method_lookup_table(ifcl, cl, objmethods_gap,
+								0) == FAIL)
+		return FAIL;
+	    ifcl = ifcl->class_extends;
+	}
     }
 
     // Update the lookup table for the extended class, if any
@@ -946,7 +1121,7 @@ add_lookup_tables(class_T *cl, class_T *extends_cl, garray_T *objmethods_gap)
 	while (pclass != NULL)
 	{
 	    if (update_member_method_lookup_table(pclass, cl,
-			objmethods_gap, pobj_method_offset, FALSE) == FAIL)
+			objmethods_gap, pobj_method_offset) == FAIL)
 		return FAIL;
 
 	    pobj_method_offset += pclass->class_obj_method_count_child;
@@ -1237,6 +1412,12 @@ ex_class(exarg_T *eap)
 	else if (STRNCMP(arg, "implements", 10) == 0
 						   && IS_WHITE_OR_NUL(arg[10]))
 	{
+	    if (!is_class)
+	    {
+		emsg(_(e_interface_cannot_use_implements));
+		goto early_ret;
+	    }
+
 	    if (ga_impl.ga_len > 0)
 	    {
 		emsg(_(e_duplicate_implements));
@@ -1377,18 +1558,25 @@ early_ret:
 		break;
 	    }
 
-	    if (!is_abstract)
+	    if (!is_class)
+		// ignore "abstract" in an interface (as all the methods in an
+		// interface are abstract.
+		p = skipwhite(pa + 8);
+	    else
 	    {
-		semsg(_(e_abstract_method_in_concrete_class), pa);
-		break;
-	    }
+		if (!is_abstract)
+		{
+		    semsg(_(e_abstract_method_in_concrete_class), pa);
+		    break;
+		}
 
-	    abstract_method = TRUE;
-	    p = skipwhite(pa + 8);
-	    if (STRNCMP(p, "def", 3) != 0 && STRNCMP(p, "static", 6) != 0)
-	    {
-		emsg(_(e_abstract_must_be_followed_by_def_or_static));
-		break;
+		abstract_method = TRUE;
+		p = skipwhite(pa + 8);
+		if (STRNCMP(p, "def", 3) != 0 && STRNCMP(p, "static", 6) != 0)
+		{
+		    emsg(_(e_abstract_must_be_followed_by_def_or_static));
+		    break;
+		}
 	    }
 	}
 
@@ -1401,6 +1589,12 @@ early_ret:
 		semsg(_(e_command_cannot_be_shortened_str), ps);
 		break;
 	    }
+
+	    if (!is_class)
+	    {
+		emsg(_(e_static_cannot_be_used_in_interface));
+		break;
+	    }
 	    has_static = TRUE;
 	    p = skipwhite(ps + 6);
 	}
@@ -1425,6 +1619,14 @@ early_ret:
 	    char_u *varname_end = NULL;
 	    type_T *type = NULL;
 	    char_u *init_expr = NULL;
+
+	    if (!is_class && *varname == '_')
+	    {
+		// private variables are not supported in an interface
+		semsg(_(e_private_variable_str_in_interface), varname);
+		break;
+	    }
+
 	    if (parse_member(eap, line, varname, has_public,
 			  &varname_end, &type_list, &type,
 			  is_class ? &init_expr: NULL) == FAIL)
@@ -1484,6 +1686,13 @@ early_ret:
 		char_u	*name = uf->uf_name;
 		int	is_new = STRNCMP(name, "new", 3) == 0;
 
+		if (!is_class && *name == '_')
+		{
+		    // private variables are not supported in an interface
+		    semsg(_(e_private_method_str_in_interface), name);
+		    func_clear_free(uf, FALSE);
+		    break;
+		}
 		if (is_new && !is_valid_constructor(uf, is_abstract,
 								has_static))
 		{
@@ -1562,7 +1771,7 @@ early_ret:
 
     // Check the "extends" class is valid.
     if (success && extends != NULL)
-	success = validate_extends_class(extends, &extends_cl);
+	success = validate_extends_class(extends, &extends_cl, is_class);
     VIM_CLEAR(extends);
 
     // Check the new object methods to make sure their access (public or
@@ -1571,9 +1780,15 @@ early_ret:
 	success = validate_extends_methods(&objmethods, extends_cl);
 
     // Check the new class and object variables are not duplicates of the
-    // variables in the extended class lineage.
+    // variables in the extended class lineage.  If an interface is extending
+    // another interface, then it can duplicate the member variables.
     if (success && extends_cl != NULL)
-	success = validate_extends_members(&objmembers, extends_cl);
+    {
+	if (is_class)
+	    success = extends_check_dup_members(&objmembers, extends_cl);
+	else
+	    success = extends_check_intf_var_type(&objmembers, extends_cl);
+    }
 
     // When extending an abstract class, make sure all the abstract methods in
     // the parent class are implemented.  If the current class is an abstract
@@ -1592,7 +1807,8 @@ early_ret:
 
 	success = validate_implements_classes(&ga_impl, intf_classes,
 					&classfunctions, &classmembers,
-					&objmethods, &objmembers);
+					&objmethods, &objmembers,
+					extends_cl);
     }
 
     // Check no function argument name is used as a class member.
@@ -2637,10 +2853,18 @@ class_instance_of(class_T *cl, class_T *other_cl)
     {
 	if (cl == other_cl)
 	    return TRUE;
-	// Check the implemented interfaces.
+	// Check the implemented interfaces and the super interfaces
 	for (int i = cl->class_interface_count - 1; i >= 0; --i)
-	    if (cl->class_interfaces_cl[i] == other_cl)
-		return TRUE;
+	{
+	    class_T	*intf = cl->class_interfaces_cl[i];
+	    while (intf != NULL)
+	    {
+		if (intf == other_cl)
+		    return TRUE;
+		// check the super interfaces
+		intf = intf->class_extends;
+	    }
+	}
     }
 
     return FALSE;