What happens with the tagged questions when a tag-synonym is cancelled?

Question

As far as I know, moderators can cancel tag synonyms and such things happen occasionally. (I've seen some threads about removal of tag synonyms here on meta.SE.)

How will the questions be tagged after the removal of the synonym? We have many questions tagged (master-tag). Some of them might have been tagged by (slave-tag) at the time when the synonym was created. Some of them might be tagged (master-tag), but they got this tag after a user explicitly typed (slave-tag) when entering the tags.¹ After removing the synonym, do all question keep the (master-tag)? Or do some of the questions get the (slave-tag)? (For example, the two types of questions mentioned before seem like a reasonable candidates.)

An additional question: Does it make a difference whether the two tags have been merged² or only synonymized?

Originally I have asked this at a local meta but this is probably a more appropriate place (and with a better chance to get an answer).

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¹Number of such posts is shown in the list of tag-synonyms.

²From what I have read here it seems that merging is independent from creating a synonym. But it is sometimes done when a tag-synonym is created. Based on the answer to the following post, it seems that there might be a difference: Need a rollback/undo for tag synonym merging.

Answer 1

Removing a tag synonym foo ⇒ bar essentially just tells the system to no longer consider the two tags the same (with bar as the master).

Within the database no changes are made to posts. Questions tagged bar will still have the tag bar. Similarly, questions tagged foo will still have that tag. Only now these questions will be displayed as such. Here it is important to remember that when the synonym foo ⇒ bar is introduced no changes are made to questions: they retain their given tags. The system is told the treat foo as if it were bar. In particular,

• a question tagged foo will be displayed — and otherwise act — as though it was tagged bar, and
• when a user tries to explicitly tag a question foo, it is instead given the bar tag.

This means that questions tagged foo before the synonym was introduced (and not edited while the synonym was in place) will have that tag after the synonym is removed. However no question created while the synonym was in place will be tagged foo.

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Merging foo into bar is the (silent) process by which the tags of questions are wholesale altered. Here, foo will be removed from all questions and replaced by bar (provided they don't already have that tag).

If foo is also made a synonym of bar, then when this synonym is removed no question will have the foo tag.