Alternatively, “Wikipedia disambiguation page” descriptions should be disallowed and removed from Wikidata. All disambiguation pages represent some actual concept in some languages, and a page’s isambiguation page status is already described by a Wikidata relation; adding these descriptions is not DRY and is only going to cause errors, such as as what I wrote above.

An alternative way:

1. Move the sitelinks and statements to another item (using gadget)
2. Merge two items
3. Restore the target item to the state after step 1

@Al12si: What is "merge.js" or where to find it? Please do include links if applicable. Thanks.

This is about https://www.wikidata.org/wiki/MediaWiki:Gadget-Merge.js. However, I do not understand what is meant by a

“disambiguation page” Wikidata item that needs to be merged (e.g., that is not really a disambiguation page)

and there is no example edit for the supposed bug.

Alternatively, “Wikipedia disambiguation page” descriptions should be disallowed and removed from Wikidata. All disambiguation pages represent some actual concept in some languages

is a controversial opinion not to be discussed here (FWIW disambiguation pages are supposed to be separated between e.g. different scripts).

Disambiguation items (items having P31 = Q4167410) are conceptually different from non-disambiguation items. An item is a disambiguation item exactly if it has this statement; it doesn’t matter whether the pages connected to it are or have been disambiguation pages. Wikidata strives to provide stable IDs, which fails if disambiguation and non-disambiguation items are merged. So the solution is not to merge the items, but move the sitelinks as necessary.

In T334249#8765366, @Aklapper wrote:

@Al12si: What is "merge.js" or where to find it? Please do include links if applicable. Thanks.

Are you serious? merge.js is a standard Wikidata applet..

In T334249#8766163, @Tacsipacsi wrote:

Disambiguation items (items having P31 = Q4167410) are conceptually different from non-disambiguation items.

They are not. A disambiguation item simply means the item was at one point considered a synonym having multiple meanings in some language. Even in the same language, the item could later be found to represent a general concept that can be expanded to a full article, in which case the item in that language ceases to be a disabmbiguation item.

When a disambiguation item on Wikidata has only one link to one Wikipedia page and that Wikipedia page ceases to be a disambiguation item, the Wikidata disambiguation item ceases to serve any function and need to be merged. If this should not be done merge.js should put up a big warning to never merge any disambiguation items — in which case this is still a UI bug in merge.js.

In T334249#8766640, @Al12si wrote:

Are you serious? merge.js is a standard Wikidata applet..

I am serious. Nobody knows everything, and it's up to you to provide sufficient context that this is about some gadget on a specific website. Thanks.

In T334249#8766640, @Al12si wrote:

merge.js is a standard Wikidata applet..

Actually it’s not even a default gadget (although it may make sense to make it one…).

In T334249#8766653, @Al12si wrote:

When a disambiguation item on Wikidata has only one link to one Wikipedia page and that Wikipedia page ceases to be a disambiguation item, the Wikidata disambiguation item ceases to serve any function and need to be merged.

It doesn’t need to be merged. Items that are about the same concept need to be merged, not items that serve no purpose. If one item is about a real concept, while the other one is a technical item (the disambiguation item), they aren’t about the same concept.

If this should not be done merge.js should put up a big warning to never merge any disambiguation items — in which case this is still a UI bug in merge.js.

This is a good idea. By the way, the UX of merge.js has several rough edges (e.g. when it does show an error/warning message, it’s often hard to understand), so it’s not surprising that there’s yet another one.

In T334249#8766976, @Tacsipacsi wrote:

Actually it’s not even a default gadget (although it may make sense to make it one…).

See T140124: Enable Merge gadget as default