patch 8.2.2080: Vim9: no proper error message for using s:var in for loop

Problem:    Vim9: no proper error message for using s:var in for loop.
Solution:   Give a specific error.

src/testdir/test_vim9_script.vim

@@ -1884,6 +1884,27 @@ def Test_for_loop_fails()
   CheckDefFailure(['for i in range(3)', 'echo 3'], 'E170:')
 enddef
 
+def Test_for_loop_script_var()
+  # cannot use s:var in a :def function
+  CheckDefFailure(['for s:var in range(3)', 'echo 3'], 'E1101:')
+
+  # can use s:var in Vim9 script, with or without s:
+  var lines =<< trim END
+    vim9script
+    var total = 0
+    for s:var in [1, 2, 3]
+      total += s:var
+    endfor
+    assert_equal(6, total)
+
+    total = 0
+    for var in [1, 2, 3]
+      total += var
+    endfor
+    assert_equal(6, total)
+  END
+enddef
+
 def Test_for_loop_unpack()
   var lines =<< trim END
       var result = []

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2080,
 /**/
     2079,
 /**/

src/vim9compile.c

@@ -6617,6 +6617,12 @@ compile_for(char_u *arg_start, cctx_T *cctx)
 		goto failed;
 	    }
 
+	    if (STRNCMP(name, "s:", 2) == 0)
+	    {
+		semsg(_(e_cannot_declare_script_variable_in_function), name);
+		goto failed;
+	    }
+
 	    // Reserve a variable to store "var".
 	    // TODO: check for type
 	    var_lvar = reserve_local(cctx, arg, varlen, FALSE, &t_any);