patch 9.0.2182: Vim9: need a way to reserve future extension

Problem:  Vim9: need a way to reserve future extension
Solution: reserve double underscore prefix for future use
          (Yegappan Lakshmanan)

related: #13238
closes: #13742

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

runtime/doc/vim9class.txt

@@ -364,6 +364,9 @@ super class.  Depending on the class where the member is used the
 corresponding class member will be used.  The type of the class member in a
 child class can be different from that in the super class.
 
+The double underscore (__) prefix for a class or object method name is
+reserved for future use.
+
 					*object-final-variable* *E1409*
 The |:final| keyword can be used to make a class or object variable a
 constant.  Examples: >

src/testdir/test_vim9_class.vim

@@ -9659,4 +9659,31 @@ def Test_const_class_object_variable()
   v9.CheckSourceFailure(lines, 'E1022: Type or initialization required', 3)
 enddef
 
+" Test for using double underscore prefix in a class/object method name.
+def Test_method_double_underscore_prefix()
+  # class method
+  var lines =<< trim END
+    vim9script
+    class A
+      static def __foo()
+        echo "foo"
+      enddef
+    endclass
+    defcompile
+  END
+  v9.CheckSourceFailure(lines, 'E1034: Cannot use reserved name __foo()', 3)
+
+  # object method
+  lines =<< trim END
+    vim9script
+    class A
+      def __foo()
+        echo "foo"
+      enddef
+    endclass
+    defcompile
+  END
+  v9.CheckSourceFailure(lines, 'E1034: Cannot use reserved name __foo()', 3)
+enddef
+
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2182,
 /**/
     2181,
 /**/

src/vim9class.c

@@ -1774,6 +1774,15 @@ early_ret:
 		break;
 	    }
 
+	    if (*p == '_' && *(p + 1) == '_')
+	    {
+		// double underscore prefix for a method name is currently
+		// reserved.  This could be used in the future to support
+		// object methods called by Vim builtin functions.
+		semsg(_(e_cannot_use_reserved_name_str), p);
+		break;
+	    }
+
 	    CLEAR_FIELD(ea);
 	    ea.cmd = line;
 	    ea.arg = p;