senses do not have to be considered, yet (covered by hike implementing T195652 ff.)

If senses are not covered by this ticket @Lydia_Pintscher then we should have a seperate ticket in the senses hike for merging senses (which I don't think we do right now)

Assumptions not (yet) explicitly mentioned in the ticket (T189129)

• lexemes with different lexical category or language cannot be merged
• merging of lemmas works like merging labels (identical ones ignored, others added to the target)

Conceptual Questions

• Do we create a new special page? Special:MergeLexemes? UX!?
• Do we create a new API endpoint? wblmergelexemes?
• What happens if one lexeme links to a form of the other lexeme? Do we abort the merge?
• What happens if a form has a subset of the representations of a form on the other lexeme? (e.g. en: colors, en-gb: colours vs only en:colors)
• What happens if a form has a subset of the grammatical features of a form on the other lexeme? (e.g. 1st person singular, nominative vs only 1st person singular)

To be tested

• What happens to statements that reference the Lexeme that got merged into the other? Does that magically work through redirects?
• Make sure statements linking to merged/"deleted" deleted forms does not explode?

Technical investigation starts here.

Item Merging

• API endpoint: Wikibase\Repo\Api\MergeItems
• Special page: Wikibase\Repo\Specials\SpecialMergeItems
• shared between API and special page: Wikibase\Repo\Interactors\ItemMergeInteractor

• ItemMergeInteractor basically does all the things
  • loads entities from ID
  • applies merge ChangeOps
  • saves
  • updates WatchList

• ChangeOpsMerge
  • generates label, description, alias, sitelink and statement ChangeOps
  • not great: validation happens sprinkled across methods (e.g. link check in generateStatementsChangeOps)