patch 8.2.4616: Vim9: Declarations in a {} block of a user command remain

Problem:    Vim9: Declarations in a {} block of a user command do not use Vim9
            rules if defined in a legacy script. (Yegappan Lakshmanan)
Solution:   Pretend the script is Vim9 script.

src/testdir/test_usercommands.vim

@@ -798,4 +798,40 @@ func Test_multibyte_in_usercmd()
   delcommand SubJapanesePeriodToDot
 endfunc
 
+" Declaring a variable in a {} uses Vim9 script rules, even when defined in a
+" legacy script.
+func Test_block_declaration_legacy_script()
+  let lines =<< trim END
+      command -range Rename {
+                     var save = @a
+                     @a = 'something'
+                     g:someExpr = @a
+                     @a = save
+                }
+  END
+  call writefile(lines, 'Xlegacy')
+  source Xlegacy
+
+  let lines =<< trim END
+      let @a = 'saved'
+      Rename
+      call assert_equal('something', g:someExpr)
+      call assert_equal('saved', @a)
+
+      let g:someExpr = 'xxx'
+      let @a = 'also'
+      Rename
+      call assert_equal('something', g:someExpr)
+      call assert_equal('also', @a)
+  END
+  call writefile(lines, 'Xother')
+  source Xother
+
+  unlet g:someExpr
+  call delete('Xlegacy')
+  call delete('Xother')
+  delcommand Rename
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab