feat(completion): add default completion hidden if there are no subcommands

The default completion is useful also in cases where there are no
subcommands, for example to provide completion for flags, and custom
completions for their values.

But when there are no subcommands, make it hidden. Simply adding it
non-hidden would result in a help section to be added just for it which
is just noise: the completion command is not the primary functionality
of the program, yet it would be prominently displayed in help output. It
would also get included among argument completions, which could be a
source of confusion.

completions.go

@@ -586,7 +586,7 @@ func checkIfFlagCompletion(finalCmd *Command, args []string, lastArg string) (*p
 // 2- c has no subcommands (to avoid creating one),
 // 3- c already has a 'completion' command provided by the program.
 func (c *Command) initDefaultCompletionCmd() {
-	if c.CompletionOptions.DisableDefaultCmd || !c.HasSubCommands() {
+	if c.CompletionOptions.DisableDefaultCmd {
 		return
 	}
 
@@ -598,6 +598,7 @@ func (c *Command) initDefaultCompletionCmd() {
 	}
 
 	haveNoDescFlag := !c.CompletionOptions.DisableNoDescFlag && !c.CompletionOptions.DisableDescriptions
+	hidden := c.CompletionOptions.HiddenDefaultCmd || !c.HasSubCommands()
 
 	completionCmd := &Command{
 		Use:   compCmdName,
@@ -607,7 +608,7 @@ See each sub-command's help for details on how to use the generated script.
 `, c.Root().Name()),
 		Args:              NoArgs,
 		ValidArgsFunction: NoFileCompletions,
-		Hidden:            c.CompletionOptions.HiddenDefaultCmd,
+		Hidden:            hidden,
 	}
 	c.AddCommand(completionCmd)
 

completions_test.go

@@ -2287,14 +2287,21 @@ func TestDefaultCompletionCmd(t *testing.T) {
 		Run:  emptyRun,
 	}
 
-	// Test that no completion command is created if there are not other sub-commands
+	// Test that default completion command is created, hidden if there are no other sub-commands
 	assertNoErr(t, rootCmd.Execute())
+	found := false
 	for _, cmd := range rootCmd.commands {
 		if cmd.Name() == compCmdName {
-			t.Errorf("Should not have a 'completion' command when there are no other sub-commands of root")
+			found = true
+			if !cmd.Hidden {
+				t.Errorf("The 'completion' command should be hidden when there are no other sub-commands of root")
+			}
 			break
 		}
 	}
+	if !found {
+		t.Errorf("Should have a 'completion' command when there are no other sub-commands of root")
+	}
 
 	subCmd := &Command{
 		Use: "sub",
@@ -2303,7 +2310,7 @@ func TestDefaultCompletionCmd(t *testing.T) {
 	rootCmd.AddCommand(subCmd)
 
 	// Test that a completion command is created if there are other sub-commands
-	found := false
+	found = false
 	assertNoErr(t, rootCmd.Execute())
 	for _, cmd := range rootCmd.commands {
 		if cmd.Name() == compCmdName {

shell_completions.md

@@ -8,7 +8,8 @@ The currently supported shells are:
 - PowerShell
 
 Cobra will automatically provide your program with a fully functional `completion` command,
-similarly to how it provides the `help` command.
+similarly to how it provides the `help` command. If there are no other subcommands, the
+default `completion` command will be hidden, but still functional.
 
 ## Creating your own completion command