diff --git a/src/dict.c b/src/dict.c
index 9c6b7d4..06f3871 100644
--- a/src/dict.c
+++ b/src/dict.c
@@ -284,11 +284,11 @@
 /*
  * Make a copy of dict "d".  Shallow if "deep" is FALSE.
  * The refcount of the new dict is set to 1.
- * See item_copy() for "copyID".
+ * See item_copy() for "top" and "copyID".
  * Returns NULL when out of memory.
  */
     dict_T *
-dict_copy(dict_T *orig, int deep, int copyID)
+dict_copy(dict_T *orig, int deep, int top, int copyID)
 {
     dict_T	*copy;
     dictitem_T	*di;
@@ -306,6 +306,8 @@
 	    orig->dv_copyID = copyID;
 	    orig->dv_copydict = copy;
 	}
+	copy->dv_type = alloc_type(top || deep ? &t_dict_any : orig->dv_type);
+
 	todo = (int)orig->dv_hashtab.ht_used;
 	for (hi = orig->dv_hashtab.ht_array; todo > 0 && !got_int; ++hi)
 	{
@@ -318,8 +320,8 @@
 		    break;
 		if (deep)
 		{
-		    if (item_copy(&HI2DI(hi)->di_tv, &di->di_tv, deep,
-							      copyID) == FAIL)
+		    if (item_copy(&HI2DI(hi)->di_tv, &di->di_tv,
+						  deep, FALSE, copyID) == FAIL)
 		    {
 			vim_free(di);
 			break;
@@ -1239,7 +1241,7 @@
     {
 	if (is_new)
 	{
-	    d1 = dict_copy(d1, FALSE, get_copyID());
+	    d1 = dict_copy(d1, FALSE, TRUE, get_copyID());
 	    if (d1 == NULL)
 		return;
 	}
diff --git a/src/eval.c b/src/eval.c
index 076ba1f..b5ccc4b 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -6160,6 +6160,7 @@
 /*
  * Make a copy of an item.
  * Lists and Dictionaries are also copied.  A deep copy if "deep" is set.
+ * "top" is TRUE for the toplevel of copy().
  * For deepcopy() "copyID" is zero for a full copy or the ID for when a
  * reference to an already copied list/dict can be used.
  * Returns FAIL or OK.
@@ -6169,6 +6170,7 @@
     typval_T	*from,
     typval_T	*to,
     int		deep,
+    int		top,
     int		copyID)
 {
     static int	recurse = 0;
@@ -6207,7 +6209,8 @@
 		++to->vval.v_list->lv_refcount;
 	    }
 	    else
-		to->vval.v_list = list_copy(from->vval.v_list, deep, copyID);
+		to->vval.v_list = list_copy(from->vval.v_list,
+							    deep, top, copyID);
 	    if (to->vval.v_list == NULL)
 		ret = FAIL;
 	    break;
@@ -6226,7 +6229,8 @@
 		++to->vval.v_dict->dv_refcount;
 	    }
 	    else
-		to->vval.v_dict = dict_copy(from->vval.v_dict, deep, copyID);
+		to->vval.v_dict = dict_copy(from->vval.v_dict,
+							    deep, top, copyID);
 	    if (to->vval.v_dict == NULL)
 		ret = FAIL;
 	    break;
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 281bd70..71632cd 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -1170,6 +1170,33 @@
     return &t_void;
 }
     static type_T *
+ret_copy(int argcount,
+	type2_T *argtypes,
+	type_T	**decl_type)
+{
+    if (argcount > 0)
+    {
+	if (argtypes[0].type_decl != NULL)
+	{
+	    if (argtypes[0].type_decl->tt_type == VAR_LIST)
+		*decl_type = &t_list_any;
+	    else if (argtypes[0].type_decl->tt_type == VAR_DICT)
+		*decl_type = &t_dict_any;
+	    else
+		*decl_type = argtypes[0].type_decl;
+	}
+	if (argtypes[0].type_curr != NULL)
+	{
+	    if (argtypes[0].type_curr->tt_type == VAR_LIST)
+		return &t_list_any;
+	    else if (argtypes[0].type_curr->tt_type == VAR_DICT)
+		return &t_dict_any;
+	}
+	return argtypes[0].type_curr;
+    }
+    return &t_void;
+}
+    static type_T *
 ret_extend(int argcount,
 	type2_T *argtypes,
 	type_T	**decl_type)
@@ -1571,7 +1598,7 @@
     {"confirm",		1, 4, FEARG_1,	    arg4_string_string_number_string,
 			ret_number,	    f_confirm},
     {"copy",		1, 1, FEARG_1,	    NULL,
-			ret_first_arg,	    f_copy},
+			ret_copy,	    f_copy},
     {"cos",		1, 1, FEARG_1,	    arg1_float_or_nr,
 			ret_float,	    FLOAT_FUNC(f_cos)},
     {"cosh",		1, 1, FEARG_1,	    arg1_float_or_nr,
@@ -1591,7 +1618,7 @@
 #endif
 			},
     {"deepcopy",	1, 2, FEARG_1,	    arg12_deepcopy,
-			ret_first_arg,	    f_deepcopy},
+			ret_copy,	    f_deepcopy},
     {"delete",		1, 2, FEARG_1,	    arg2_string,
 			ret_number_bool,    f_delete},
     {"deletebufline",	2, 3, FEARG_1,	    arg3_buffer_lnum_lnum,
@@ -3297,7 +3324,7 @@
     static void
 f_copy(typval_T *argvars, typval_T *rettv)
 {
-    item_copy(&argvars[0], rettv, FALSE, 0);
+    item_copy(&argvars[0], rettv, FALSE, TRUE, 0);
 }
 
 /*
@@ -3439,7 +3466,7 @@
     else
     {
 	copyID = get_copyID();
-	item_copy(&argvars[0], rettv, TRUE, noref == 0 ? copyID : 0);
+	item_copy(&argvars[0], rettv, TRUE, TRUE, noref == 0 ? copyID : 0);
     }
 }
 
diff --git a/src/evalvars.c b/src/evalvars.c
index f88be3e..b343606 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -3695,24 +3695,7 @@
     free_tv_arg = FALSE;
 
     if (vim9script && type != NULL)
-    {
-	if (type->tt_type == VAR_DICT && dest_tv->vval.v_dict != NULL)
-	{
-	    if (dest_tv->vval.v_dict->dv_type != type)
-	    {
-		free_type(dest_tv->vval.v_dict->dv_type);
-		dest_tv->vval.v_dict->dv_type = alloc_type(type);
-	    }
-	}
-	else if (type->tt_type == VAR_LIST && dest_tv->vval.v_list != NULL)
-	{
-	    if (dest_tv->vval.v_list->lv_type != type)
-	    {
-		free_type(dest_tv->vval.v_list->lv_type);
-		dest_tv->vval.v_list->lv_type = alloc_type(type);
-	    }
-	}
-    }
+	set_tv_type(dest_tv, type);
 
     // ":const var = value" locks the value
     // ":final var = value" locks "var"
diff --git a/src/list.c b/src/list.c
index ff7d5ab..86c1679 100644
--- a/src/list.c
+++ b/src/list.c
@@ -1015,7 +1015,7 @@
 
     if (make_copy)
     {
-	l = list_copy(l, TRUE, get_copyID());
+	l = list_copy(l, TRUE, TRUE, get_copyID());
 	rettv->vval.v_list = l;
 	if (l == NULL)
 	    return;
@@ -1102,7 +1102,7 @@
     if (l1 == NULL)
 	l = list_alloc();
     else
-	l = list_copy(l1, FALSE, 0);
+	l = list_copy(l1, FALSE, TRUE, 0);
     if (l == NULL)
 	return FAIL;
     tv->v_type = VAR_LIST;
@@ -1200,11 +1200,11 @@
 /*
  * Make a copy of list "orig".  Shallow if "deep" is FALSE.
  * The refcount of the new list is set to 1.
- * See item_copy() for "copyID".
+ * See item_copy() for "top" and "copyID".
  * Returns NULL when out of memory.
  */
     list_T *
-list_copy(list_T *orig, int deep, int copyID)
+list_copy(list_T *orig, int deep, int top, int copyID)
 {
     list_T	*copy;
     listitem_T	*item;
@@ -1216,7 +1216,7 @@
     copy = list_alloc();
     if (copy != NULL)
     {
-	copy->lv_type = alloc_type(orig->lv_type);
+	copy->lv_type = alloc_type(top || deep ? &t_list_any: orig->lv_type);
 	if (copyID != 0)
 	{
 	    // Do this before adding the items, because one of the items may
@@ -1233,7 +1233,8 @@
 		break;
 	    if (deep)
 	    {
-		if (item_copy(&item->li_tv, &ni->li_tv, deep, copyID) == FAIL)
+		if (item_copy(&item->li_tv, &ni->li_tv,
+						  deep, FALSE, copyID) == FAIL)
 		{
 		    vim_free(ni);
 		    break;
@@ -2701,11 +2702,11 @@
     }
     l2 = argvars[1].vval.v_list;
     if ((is_new || !value_check_lock(l1->lv_lock, arg_errmsg, TRUE))
-	    && l2 != NULL)
+								 && l2 != NULL)
     {
 	if (is_new)
 	{
-	    l1 = list_copy(l1, FALSE, get_copyID());
+	    l1 = list_copy(l1, FALSE, TRUE, get_copyID());
 	    if (l1 == NULL)
 		return;
 	}
diff --git a/src/proto/dict.pro b/src/proto/dict.pro
index d6cf152..7db011d 100644
--- a/src/proto/dict.pro
+++ b/src/proto/dict.pro
@@ -12,7 +12,7 @@
 dictitem_T *dictitem_alloc(char_u *key);
 void dictitem_remove(dict_T *dict, dictitem_T *item);
 void dictitem_free(dictitem_T *item);
-dict_T *dict_copy(dict_T *orig, int deep, int copyID);
+dict_T *dict_copy(dict_T *orig, int deep, int top, int copyID);
 int dict_wrong_func_name(dict_T *d, typval_T *tv, char_u *name);
 int dict_add(dict_T *d, dictitem_T *item);
 int dict_add_number(dict_T *d, char *key, varnumber_T nr);
diff --git a/src/proto/eval.pro b/src/proto/eval.pro
index abb9c56..8be5885 100644
--- a/src/proto/eval.pro
+++ b/src/proto/eval.pro
@@ -69,7 +69,7 @@
 int eval_isnamec1(int c);
 int eval_isdictc(int c);
 int handle_subscript(char_u **arg, char_u *name_start, typval_T *rettv, evalarg_T *evalarg, int verbose);
-int item_copy(typval_T *from, typval_T *to, int deep, int copyID);
+int item_copy(typval_T *from, typval_T *to, int deep, int top, int copyID);
 void echo_one(typval_T *rettv, int with_space, int *atstart, int *needclr);
 void ex_echo(exarg_T *eap);
 void ex_echohl(exarg_T *eap);
diff --git a/src/proto/list.pro b/src/proto/list.pro
index 5b7414b..468775e 100644
--- a/src/proto/list.pro
+++ b/src/proto/list.pro
@@ -39,7 +39,7 @@
 int list_concat(list_T *l1, list_T *l2, typval_T *tv);
 list_T *list_slice(list_T *ol, long n1, long n2);
 int list_slice_or_index(list_T *list, int range, varnumber_T n1_arg, varnumber_T n2_arg, int exclusive, typval_T *rettv, int verbose);
-list_T *list_copy(list_T *orig, int deep, int copyID);
+list_T *list_copy(list_T *orig, int deep, int top, int copyID);
 void vimlist_remove(list_T *l, listitem_T *item, listitem_T *item2);
 char_u *list2string(typval_T *tv, int copyID, int restore_copyID);
 int list_join(garray_T *gap, list_T *l, char_u *sep, int echo_style, int restore_copyID, int copyID);
diff --git a/src/proto/vim9type.pro b/src/proto/vim9type.pro
index 61e38e0..fb3cb8d 100644
--- a/src/proto/vim9type.pro
+++ b/src/proto/vim9type.pro
@@ -2,6 +2,7 @@
 void clear_type_list(garray_T *gap);
 type_T *alloc_type(type_T *type);
 void free_type(type_T *type);
+void set_tv_type(typval_T *tv, type_T *type);
 type_T *get_list_type(type_T *member_type, garray_T *type_gap);
 type_T *get_dict_type(type_T *member_type, garray_T *type_gap);
 type_T *alloc_func_type(type_T *ret_type, int argcount, garray_T *type_gap);
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index e3606d8..73b3310 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -484,7 +484,7 @@
           ->copy()
           ->copy()
   END
-  v9.CheckDefFailure(lines, 'E1012:', 2)
+  v9.CheckDefExecFailure(lines, 'E1012:', 4)
 
   lines =<< trim END
       var x: any
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 248887f..f7a5006 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -720,6 +720,31 @@
   res->assert_equal(6)
 
   dl = deepcopy([1, 2, 3], true)
+
+  # after a copy() the type can change, but not the item itself
+  var nl: list<number> = [1, 2]
+  assert_equal([1, 2, 'x'], nl->copy()->extend(['x']))
+
+  var lines =<< trim END
+      var nll: list<list<number>> = [[1, 2]]
+      nll->copy()[0]->extend(['x'])
+  END
+  v9.CheckDefExecAndScriptFailure(lines, 'E1013: Argument 2: type mismatch, expected list<number> but got list<string> in extend()')
+
+  var nd: dict<number> = {a: 1, b: 2}
+  assert_equal({a: 1, b: 2, c: 'x'}, nd->copy()->extend({c: 'x'}))
+  lines =<< trim END
+      var ndd: dict<dict<number>> = {a: {x: 1, y: 2}}
+      ndd->copy()['a']->extend({z: 'x'})
+  END
+  v9.CheckDefExecAndScriptFailure(lines, 'E1013: Argument 2: type mismatch, expected dict<number> but got dict<string> in extend()')
+
+  # after a deepcopy() the item type can also change
+  var nll: list<list<number>> = [[1, 2]]
+  assert_equal([1, 2, 'x'], nll->deepcopy()[0]->extend(['x']))
+
+  var ndd: dict<dict<number>> = {a: {x: 1, y: 2}}
+  assert_equal({x: 1, y: 2, z: 'x'}, ndd->deepcopy()['a']->extend({z: 'x'}))
 enddef
 
 def Test_count()
diff --git a/src/version.c b/src/version.c
index 2c62367..e6edb17 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4286,
+/**/
     4285,
 /**/
     4284,
diff --git a/src/vim9execute.c b/src/vim9execute.c
index a45d7c5..cac8c51 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -4412,7 +4412,8 @@
 								       == NULL)
 		    {
 			SOURCING_LNUM = iptr->isn_lnum;
-			semsg(_(e_key_not_present_in_dictionary), iptr->isn_arg.string);
+			semsg(_(e_key_not_present_in_dictionary),
+							 iptr->isn_arg.string);
 			goto on_error;
 		    }
 		    // Put the dict used on the dict stack, it might be used by
@@ -4531,21 +4532,7 @@
 		break;
 
 	    case ISN_SETTYPE:
-		{
-		    checktype_T *ct = &iptr->isn_arg.type;
-
-		    tv = STACK_TV_BOT(-1);
-		    if (tv->v_type == VAR_DICT && tv->vval.v_dict != NULL)
-		    {
-			free_type(tv->vval.v_dict->dv_type);
-			tv->vval.v_dict->dv_type = alloc_type(ct->ct_type);
-		    }
-		    else if (tv->v_type == VAR_LIST && tv->vval.v_list != NULL)
-		    {
-			free_type(tv->vval.v_list->lv_type);
-			tv->vval.v_list->lv_type = alloc_type(ct->ct_type);
-		    }
-		}
+		set_tv_type(STACK_TV_BOT(-1), iptr->isn_arg.type.ct_type);
 		break;
 
 	    case ISN_2BOOL:
diff --git a/src/vim9type.c b/src/vim9type.c
index f2a69d0..f184171 100644
--- a/src/vim9type.c
+++ b/src/vim9type.c
@@ -102,6 +102,71 @@
     vim_free(type);
 }
 
+/*
+ * Return TRUE if "type" is to be recursed into for setting the type.
+ */
+    static int
+set_tv_type_recurse(type_T *type)
+{
+    return type->tt_member != NULL
+		&& (type->tt_member->tt_type == VAR_DICT
+				       || type->tt_member->tt_type == VAR_LIST)
+		&& type->tt_member->tt_member != NULL
+		&& type->tt_member->tt_member != &t_any
+		&& type->tt_member->tt_member != &t_unknown;
+}
+
+/*
+ * Set the type of "tv" to "type" if it is a list or dict.
+ */
+    void
+set_tv_type(typval_T *tv, type_T *type)
+{
+    if (tv->v_type == VAR_DICT && tv->vval.v_dict != NULL)
+    {
+	dict_T *d = tv->vval.v_dict;
+
+	if (d->dv_type != type)
+	{
+	    free_type(d->dv_type);
+	    d->dv_type = alloc_type(type);
+	    if (set_tv_type_recurse(type))
+	    {
+		int		todo = (int)d->dv_hashtab.ht_used;
+		hashitem_T	*hi;
+		dictitem_T	*di;
+
+		for (hi = d->dv_hashtab.ht_array; todo > 0; ++hi)
+		{
+		    if (!HASHITEM_EMPTY(hi))
+		    {
+			--todo;
+			di = HI2DI(hi);
+			set_tv_type(&di->di_tv, type->tt_member);
+		    }
+		}
+	    }
+	}
+    }
+    else if (tv->v_type == VAR_LIST && tv->vval.v_list != NULL)
+    {
+	list_T *l = tv->vval.v_list;
+
+	if (l->lv_type != type)
+	{
+	    free_type(l->lv_type);
+	    l->lv_type = alloc_type(type);
+	    if (l->lv_first != &range_list_item && set_tv_type_recurse(type))
+	    {
+		listitem_T	*li;
+
+		FOR_ALL_LIST_ITEMS(l, li)
+		    set_tv_type(&li->li_tv, type->tt_member);
+	    }
+	}
+    }
+}
+
     type_T *
 get_list_type(type_T *member_type, garray_T *type_gap)
 {