Add HasCustomFlagCompletion method to Command struct for flag completion validation

command.go

@@ -1607,6 +1607,17 @@ func (c *Command) HasSubCommands() bool {
 	return len(c.commands) > 0
 }
 
+// IsAvailableCommand determines if a command is available as a non-help command
+// (this includes all non deprecated/hidden commands).
+func (c *Command) HasCustomFlagCompletion() bool {
+
+	if c.ValidArgsFunction == nil {
+		return false
+	}
+
+	return c.AllowCustomFlagCompletions
+}
+
 // IsAvailableCommand determines if a command is available as a non-help command
 // (this includes all non deprecated/hidden commands).
 func (c *Command) IsAvailableCommand() bool {