Change 1007693 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):

[mediawiki/core@master] Make Message and MessageValue compatible

https://gerrit.wikimedia.org/r/1007693

The change to implement this is now ready for review: Make Message and MessageValue compatible.

Adding the MessageSpecifier interface to the MessageValue class seems to be pretty uneventful. I only had to make some updates to the StatusValue class, which previously accepted both MessageSpecifier and MessageValue and converted between them, and some of that code would go down the wrong path after the change.

The bigger change is changing how the Message class stores parameters internally to match the MessageValue class: previously it used assoc arrays, e.g. [ 'num' => 123 ], which are replaced by MessageParam value objects. Code that uses public methods like ->numParams() won't be affected by this change, but I found a few places in core and extensions that assumed the previous behavior. I updated them: https://gerrit.wikimedia.org/r/q/topic:message-param but it's worth noting that this may affect other code that wasn't covered by tests that'd let me find them. The old behavior was never documented, so I don't consider this to be a breaking change to a public interface.

A few other patches have been split off, and merged already:

• StatusValue: Add a getter for MessageSpecifier list
  • This one turned out to be a pretty good idea by itself, and I wrote a lot of patches for core and extensions to use the new method, StatusValue::getMessages(), making the code shorter, simpler and resolving TODO comments: https://gerrit.wikimedia.org/r/q/topic:StatusValue-getMessages
• Make MessageValue implement JsonDeserializable
• Document more methods that accept MessageSpecifier, not just Message
• language: Improve behavior of RawMessage used as MessageSpecifier
• Namespace MessageSpecifier under Wikimedia\Message\

Thanks to everyone who reviewed those!