https://github.com/vim-jp/issues/issues/991

filter-cmd-completion.patch

diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 439467cf1..9dcbe4208 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -3864,7 +3864,6 @@ set_one_cmd_context(
 	case CMD_cfdo:
 	case CMD_confirm:
 	case CMD_debug:
-	case CMD_filter:
 	case CMD_folddoclosed:
 	case CMD_folddoopen:
 	case CMD_hide:
@@ -3889,6 +3888,16 @@ set_one_cmd_context(
 	case CMD_windo:
 	    return arg;
 
+	case CMD_filter:
+	    if (*arg != NUL)
+		arg = skip_vimgrep_pat(arg, NULL, NULL);
+	    if (arg == NULL || *arg == NUL)
+	    {
+		xp->xp_context = EXPAND_NOTHING;
+		return NULL;
+	    }
+	    return skipwhite(arg);
+
 #ifdef FEAT_CMDL_COMPL
 # ifdef FEAT_SEARCH_EXTRA
 	case CMD_match:
diff --git a/src/testdir/test_filter_cmd.vim b/src/testdir/test_filter_cmd.vim
index 0bbd905c8..5aa5fa64d 100644
--- a/src/testdir/test_filter_cmd.vim
+++ b/src/testdir/test_filter_cmd.vim
@@ -52,3 +52,25 @@ func Test_filter_fails()
   call assert_fails('filter! /pat/', 'E476:')
   call assert_fails('filter! /pat/ asdf', 'E492:')
 endfunc
+
+function s:complete_filter_cmd(filtcmd)
+  let keystroke = "\<TAB>\<C-R>=execute('let cmdline = getcmdline()')\<CR>\<C-C>"
+  let cmdline = ''
+  call feedkeys(':' . a:filtcmd . keystroke, 'ntx')
+  return cmdline
+endfunction
+
+func Test_filter_cmd_completion()
+  " Do not complete pattern
+  call assert_equal("filter \t", s:complete_filter_cmd('filter '))
+  call assert_equal("filter pat\t", s:complete_filter_cmd('filter pat'))
+  call assert_equal("filter /pat\t", s:complete_filter_cmd('filter /pat'))
+  call assert_equal("filter /pat/\t", s:complete_filter_cmd('filter /pat/'))
+
+  " Complete after string pattern
+  call assert_equal('filter pat print', s:complete_filter_cmd('filter pat pri'))
+
+  " Complete after regexp pattern
+  call assert_equal('filter /pat/ print', s:complete_filter_cmd('filter /pat/ pri'))
+  call assert_equal('filter #pat# print', s:complete_filter_cmd('filter #pat# pri'))
+endfunc