2.12.7 regression wrt implicit conversions in super-class companion objects

[japgolly]

scalajs-react doesn't compile with 2.12.7. (Nor do any downstream projects).

Here is Travis compiling with both 2.12.6 and 2.12.7:
https://travis-ci.org/japgolly/scalajs-react/builds/434352061

The error seems to be that this implicit conversion here is no longer being considered.

[Atry]

Maybe a duplicate of #11068

[joroKr21]

Maybe a duplicate of #11068

Unlikely, the bug fix that caused #11068 is not in 2.12

[adriaanm]

Looks like it actually is scala/scala#7067 this time:

With a locally-built scala (https://github.com/scala/scala/compare/v2.12.7...adriaanm:revert-7067?expand=1), sbt test:compile passes for: https://github.com/japgolly/scalajs-react/compare/master...adriaanm:revert-7067?expand=1

[SethTisue]

/cc @milessabin

[milessabin]

Ack ... I'm travelling but should be able to take a look over the weekend.

[japgolly]

Oh btw, take my title of this issue with a healthy serve of salt. It's a guess. The variance in the HKT also seems like something that could easily slip past the community build. Or maybe something else.

[milessabin]

I think I can see what's going on here ... stay tuned.

[milessabin]

PR here: scala/scala#7295.

scalajs-react compiles locally for me with that applied.