RepresentableSuite.scala doesn't compile in Scala 2.13 community build

[SethTisue]

[cats] Running "test" in: testsJVM
[cats] [info] Compiling 88 Scala sources to /Users/tisue/community-builds.213/target-0.9.12/project-builds/cats-f6470f629c8fd47a8d425f3562a7e38f7d16ca33/tests/.jvm/target/scala-2.13.0-pre-ba03b6d/test-classes ...
[cats] [error] /Users/tisue/community-builds.213/target-0.9.12/project-builds/cats-f6470f629c8fd47a8d425f3562a7e38f7d16ca33/tests/src/test/scala/cats/tests/RepresentableSuite.scala:63:58: could not find implicit value for parameter iso: cats.laws.discipline.SemigroupalTests.Isomorphisms[[β$5$]String => β$5$]
[cats] [error]     checkAll("String => ?", MonadTests[String => ?].monad[String, String, String])
[cats] [error]                                                          ^

this is on 2.13.0-pre-ba03b6d

not sure what to make of this; the code compiles on 2.13.0-M4, so I suppose this is due to some post-M4 change in scala/scala

tracking this down could be tricky. 277 PRs (!) have been merged, as of scala/scala@ba03b6d, since M4, and Cats has a bunch of dependencies so it's not easy to build and test everything against 2.13.0-pre-ba03b6d

can someone who knows the cats code explain what might be going wrong here? and then I can ask the Scala team to take a look and see what PRs might explain it?

worst comes to worst, I can use the community build to bisect what Scala PR this regressed in, but doing that would take a long time

[kailuowang]

@SethTisue when you said the code compiles on 2.13.0-M4, did you use master or the 1.2.x. branch?
In several tests like this one right above we had to manually call the implicit method in the companion to provide a local implicit val in scope to help scalac. I am not sure if anyone ever figured out when it's needed and why (maybe @ceedubs ?)
How about we give it a try first? if it ends up fixing the issue we just know that the scalac bug's (not sure if it's identified and reported) behavior changed a bit ( we don't know if any of the existing manual ones are no longer needed because of this change).

[SethTisue]

did you use master or the 1.2.x. branch?

uh, I don't remember, but they both currently compile on 2.13.0-M4.

(as an aside, I had hoped the problem might be reproducible outside dbuild with ; ++2.13.0-pre-329c477! ; set every scalaBinaryVersion := "2.13.0-M4" ; testsJVM/Test/compile but no, we've made binary-incompatible collections changes since M4 so that bombs)

scala/community-builds@scala/community-build@ea62fa4 makes this a bit more convenient to troubleshoot using dbuild

I'll poke at it using your suggestion about altering the test in question

[SethTisue]

PR: #2359

[SethTisue]

cats is green in 2.13 community build, has been for a while