patch 9.0.1824: Vim9: private members may be modifiable

Problem:  Vim9: private members may be modifiable
Solution: prevent modification for def function

closes: #12963

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

src/testdir/test_vim9_class.vim

@@ -3653,4 +3653,37 @@ def Test_dup_member_variable()
   v9.CheckScriptFailure(lines, 'E1369: Duplicate member: val')
 enddef
 
+" Test for accessing a private member outside a class in a def function
+def Test_private_member_access_outside_class()
+  # private object member variable
+  var lines =<< trim END
+    vim9script
+    class A
+      this._val = 10
+      def GetVal(): number
+        return this._val
+      enddef
+    endclass
+    def T()
+      var a = A.new()
+      a._val = 20
+    enddef
+    T()
+  END
+  v9.CheckScriptFailure(lines, 'E1333: Cannot access private member: _val')
+
+  # private class member variable
+  lines =<< trim END
+    vim9script
+    class A
+      static _val: number = 10
+    endclass
+    def T()
+      A._val = 20
+    enddef
+    T()
+  END
+  v9.CheckScriptFailure(lines, 'E1333: Cannot access private member: _val')
+enddef
+
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker

src/version.c

@@ -699,6 +699,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1824,
 /**/
     1823,
 /**/

src/vim9compile.c

@@ -1866,6 +1866,15 @@ compile_lhs(
 	{
 	    // for an object or class member get the type of the member
 	    class_T *cl = lhs->lhs_type->tt_class;
+	    // If it is private member variable, then accessing it outside the
+	    // class is not allowed.
+	    if (*(after + 1) == '_' && !inside_class(cctx, cl))
+	    {
+		char_u *m_name = vim_strnsave(after + 1, lhs->lhs_end - after);
+		semsg(_(e_cannot_access_private_member_str), m_name);
+		vim_free(m_name);
+		return FAIL;
+	    }
 	    lhs->lhs_member_type = class_member_type(cl, after + 1,
 					   lhs->lhs_end, &lhs->lhs_member_idx);
 	    if (lhs->lhs_member_idx < 0)