Document migration steps for changed rules for given prioritisation under 3.6 about scala3 HOT 6 OPEN

The main issue with this change is that it's a problem that manifests down to the application level. We cannot simply just silence the warnings at the library level. Meaning that any library that experiences this, now needs its own migration guide for its users.

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@lenguyenthanh We did tests in the open CB and there was only a tiny minority of projects that broke (something like 5 out of 1500). I was myself very surprised by this and that was the main reason why we decided that we could do the change. After all the change is a huge improvement, without it you cannot really do serious typeclass-based development.

Most of the breakage traced back to Reactive Mongo which is also the root error for Lichess. There were more warnings before but since our latest fixes to avoid spurious warnings it should be much better.

We should do a careful analysis of ReactiveMongo and advise downstream projects of possible breakages. The implicits in Reactive Mongo's BSON handling that cause the switch in priority are unfortunately extremely complex and inscrutable (at least to my eyes).

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I "successful" migrated lila to 3.5.0-RC3 with -source:3.6-migration: lichess-org/lila#15664. But I'm quite afraid that #19300 will cause a lot of trouble to the community/industry as @soronpo said above.

I believe that there are a lot of projects out there depend on the rule of choosing the most specific implicit (before #19300). Which is kinda intuitive in OOP sense.

I really think that We have to have some migration guide before 3.5 release.

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cc @odersky

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@bracevac this would be covered by scala/scala-lang#1675, right?

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@lenguyenthanh We did tests in the open CB and there was only a tiny minority of projects that broke (something like 5 out of 1500). I was myself very surprised by this and that was the main reason why we decided that we could do the change. After all the change is a huge improvement, without it you cannot really do serious typeclass-based development.

I'm 100% behind this change. It does indeed improve Scala given semantic and I'm very optimistic that this change combining with scala/improvement-proposals#81 It'll easier for new comers understand and use contextual abstraction. Which is one of the power of Scala IMHO.

Most of the breakage traced back to Reactive Mongo which is also the root error for Lichess. There were more warnings before but since our latest fixes to avoid spurious warnings it should be much better.

We should do a careful analysis of ReactiveMongo and advise downstream projects of possible breakages. The implicits in Reactive Mongo's BSON handling that cause the switch in priority are unfortunately extremely complex and inscrutable (at least to my eyes).

My concern is mostly about the migration works for the industry. If the errors is "obvious" like in reactive-mongo, We (the industry) can solve it (maybe somewhat difficult but it's solvable, I solved it in lila by reading the PR and reactive-mongo code). But sometime, We make it compiles, no warning anymore, but the business logic is reversed. And I think this kind of errors is the one We should focus on the migration documentation.

Here are some issues We have had after 3.5.0-RC4 (with given prioritisation change) migrations:

• Read wrong format from database
• Write wrong data to the database

p/s: I agree that the way reactive-mongo and lila using implicit and given are over complicated (I would say "abusing") but I guess that not to abnormal in the industry.

edited to fix typo and add some more information.

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