patch 8.2.4500: Vim9: can declare a global variable on the command line

Problem:    Vim9: can declare a global variable on the command line.
Solution:   Disallow declaring a variable on the command line. (closes #9881)
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index 68178cf..87acff6 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -3326,34 +3326,50 @@
 func Test_no_redraw_when_restoring_cpo()
   CheckScreendump
   CheckFeature timers
+  call Run_test_no_redraw_when_restoring_cpo()
+endfunc
 
-  let lines =<< trim END
+def Run_test_no_redraw_when_restoring_cpo()
+  var lines =<< trim END
     vim9script
     export def Func()
     enddef
   END
-  call mkdir('Xdir/autoload', 'p')
-  call writefile(lines, 'Xdir/autoload/script.vim')
+  mkdir('Xdir/autoload', 'p')
+  writefile(lines, 'Xdir/autoload/script.vim')
 
-  let lines =<< trim END
+  lines =<< trim END
       vim9script
       set cpo+=M
       exe 'set rtp^=' .. getcwd() .. '/Xdir'
       au CmdlineEnter : ++once timer_start(0, (_) => script#Func())
       setline(1, 'some text')
   END
-  call writefile(lines, 'XTest_redraw_cpo')
-  let buf = g:RunVimInTerminal('-S XTest_redraw_cpo', {'rows': 6})
-  call term_sendkeys(buf, "V:")
-  call VerifyScreenDump(buf, 'Test_vim9_no_redraw', {})
+  writefile(lines, 'XTest_redraw_cpo')
+  var buf = g:RunVimInTerminal('-S XTest_redraw_cpo', {'rows': 6})
+  term_sendkeys(buf, "V:")
+  g:VerifyScreenDump(buf, 'Test_vim9_no_redraw', {})
 
-  " clean up
-  call term_sendkeys(buf, "\<Esc>u")
-  call g:StopVimInTerminal(buf)
-  call delete('XTest_redraw_cpo')
-  call delete('Xdir', 'rf')
+  # clean up
+  term_sendkeys(buf, "\<Esc>u")
+  g:StopVimInTerminal(buf)
+  delete('XTest_redraw_cpo')
+  delete('Xdir', 'rf')
+enddef
+
+func Test_reject_declaration()
+  CheckScreendump
+  call Run_test_reject_declaration()
 endfunc
 
+def Run_test_reject_declaration()
+  var buf = g:RunVimInTerminal('', {'rows': 6})
+  term_sendkeys(buf, ":vim9cmd var x: number\<CR>")
+  g:VerifyScreenDump(buf, 'Test_vim9_reject_declaration', {})
+
+  # clean up
+  g:StopVimInTerminal(buf)
+enddef
 
 def Test_unset_any_variable()
   var lines =<< trim END