fix: validate that User/Role flags and UserIds/RoleIds lists are mutually exclusive

This change adds validation to SendMessageAsync which checks that the User flag is mutually exclusive with the list of UserIds, and that the Role flag is also mutually exclusive with the list of RoleIds

src/Discord.Net.Rest/Entities/Channels/ChannelHelper.cs

@@ -172,6 +172,22 @@ namespace Discord.Rest
             Preconditions.AtMost(allowedMentions?.RoleIds?.Count ?? 0, 100, nameof(allowedMentions.RoleIds), "A max of 100 role Ids are allowed.");
             Preconditions.AtMost(allowedMentions?.UserIds?.Count ?? 0, 100, nameof(allowedMentions.UserIds), "A max of 100 user Ids are allowed.");
 
+            // check that user flag and user Id list are exclusive, same with role flag and role Id list
+            if (allowedMentions != null && allowedMentions.AllowedTypes.HasValue)
+            {
+                if (allowedMentions.AllowedTypes.Value.HasFlag(AllowedMentionTypes.Users) &&
+                    allowedMentions.UserIds != null && allowedMentions.UserIds.Count > 0)
+                {
+                    throw new ArgumentException("The Users flag is mutually exclusive with the list of User Ids.", nameof(allowedMentions));
+                }
+
+                if (allowedMentions.AllowedTypes.Value.HasFlag(AllowedMentionTypes.Roles) &&
+                    allowedMentions.RoleIds != null && allowedMentions.RoleIds.Count > 0)
+                {
+                    throw new ArgumentException("The Roles flag is mutually exclusive with the list of Role Ids.", nameof(allowedMentions));
+                }
+            }
+
             var args = new CreateMessageParams(text) { IsTTS = isTTS, Embed = embed?.ToModel(), AllowedMentions = allowedMentions?.ToModel() };
             var model = await client.ApiClient.CreateMessageAsync(channel.Id, args, options).ConfigureAwait(false);
             return RestUserMessage.Create(client, channel, client.CurrentUser, model);