make Tagged a universal trait so tagging value classes works

[kwark]

This fixes issue #644

I also ran mima and it did not find any binary incompatible changes:

[IJ]> mimaReportBinaryIssues
[info] Resolving com.chuusai#shapeless_2.12;2.3.2 ...
[info] Resolving org.scala-lang#scala-library;2.12.0 ...
[info] Resolving org.typelevel#macro-compat_2.12;1.1.1 ...
[info] Resolving com.chuusai#shapeless_sjs0.6_2.12;2.3.2 ...
[info] Resolving org.scala-lang#scala-library;2.12.0 ...
[info] Resolving org.scala-js#scalajs-library_2.12;0.6.13 ...
[info] Resolving org.typelevel#macro-compat_2.12;1.1.1 ...
[info] Compiling 2 Scala sources to /Users/peter/dev/shapeless/core/jvm/target/scala-2.12/classes...
[info] Compiling 2 Scala sources to /Users/peter/dev/shapeless/core/js/target/scala-2.12/classes...
[info] Compiling 13 Scala sources to /Users/peter/dev/shapeless/core/jvm/target/scala-2.12/classes...
[info] Compiling 13 Scala sources to /Users/peter/dev/shapeless/core/js/target/scala-2.12/classes...
[info] Compiling 85 Scala sources to /Users/peter/dev/shapeless/core/jvm/target/scala-2.12/classes...
[info] Compiling 85 Scala sources to /Users/peter/dev/shapeless/core/js/target/scala-2.12/classes...
[info] coreJVM: found 0 potential binary incompatibilities while checking against com.chuusai:shapeless_2.12:2.3.2  (filtered 6)
[info] root: previous-artifact not set, not analyzing binary compatibility
[info] coreJS: found 0 potential binary incompatibilities while checking against com.chuusai:shapeless_sjs0.6_2.12:2.3.2  (filtered 6)
[success] Total time: 49 s, completed Dec 31, 2017 9:31:01 AM

[codecov-io]

Codecov Report

Merging #801 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #801   +/-   ##
=======================================
  Coverage   87.99%   87.99%           
=======================================
  Files          64       64           
  Lines        1499     1499           
  Branches        5        5           
=======================================
  Hits         1319     1319           
  Misses        180      180

Impacted Files	Coverage Δ
core/src/main/scala/shapeless/typeoperators.scala	0% <ø> (ø)	⬆️

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[milessabin]

Thanks for this ... I'm literally about to push the button on releasing 2.3.3, so this has missed the boat unfortunately.

This looks good, but we've made several attempts to improve things in this area before and they've all turned out to have downsides of one sort or another. I have a vague feeling that we've actually tried exactly this change previously ... would you mind going through the related PRs and issues and see if you can work out where things stand?

[kwark]

I have been going through some related PRs:

• The change for Tagged from a type alias to a trait was introduced by Change type tags to a trait  #20. The trait was not made universal because there was/is indeed no difference when looking at the decompiled bytecode with javap.
• Runtime exception if the value of record field is Any or AnyRef #44 is still an issue, even when Tagged is made a universal trait.
• tag[Tag](value) doesnt work with explicitly declared target type #557 is of course also still an issue, because it is a bug in scalac

I don't find any evidence that this specific change has been tried previously.
I do acknowledge that this kind of change might inadvertently impact other areas of the shapeless codebase.

@milessabin I'll be happy to check other PRs/issues. Do you have any specific in mind?

[joroKr21]

Needs rebase. Is there any reason not to do this? Looks like a no-brainer to me.

[milessabin]

Is there any reason not to do this? Looks like a no-brainer to me.

Issues with 2.10 maybe?

[joroKr21]

We dropped 2.10 🎉