The URLs of merged topics do not forward to the updated topic URL

[arielmeadow]

When you merge thread A into thread B, any URLs for thread A break.

This causes an issue for forum members participating in topic A who are following along via email notification.

Here's the flow:

1. Member responds to topic A and opts to receive email notfications
2. Other members respond. Member is sent email notifications about responses to topic A.
3. A moderator merges topic A into topic B
4. Member goes to check email, sees notifications about topic A, and clicks URL in notification email.
5. Member is directed to topic A's URL, which is now a 404. Member does not know that topic A is now part of topic B.

To keep notifications functioning effectively, it makes sense to have merged topic A's URL forward to topic B's URL.

[johnjamesjacoby]

This is anticipated behavior for now. When you merge two topics, it's a literal merging of data together. I know that existing forums have gotten around this problem by cloning topics, but that seems like a poor approach.

Moving to future release to ruminate.

[netweb]

When merging a topic the old topic is assigned a postmeta key _wp_old_slug

See ​http://developer.wordpress.org/reference/functions/wp_old_slug_redirect/

I think we can use this, as we are now using the reply ID for the reply slug and if WP 301 redirects the old url to new url for us along with #2424 to redirect single replies to the parent topic we would be good to go.

Related: #2424 #2528