Scala.js 1.3.1 with scalajs-bundler breaks Circe

See the following repository for full example: https://github.com/kamilkloch/x-monitor-webpack.git

sbt
project frontend
fastOptJS / webpack

Open frontend/index-dev.html, show console.

With Scala.js 1.3.1 web console reads

{
   "name" : "Bob"
}
io.circe.FailedCursor@1
Left(DecodingFailure(Attempt to decode value on failed cursor, List(DownField(name))))

With Scala.js 1.3.0 it works:

{
  "name" : "Bob"
}
io.circe.cursor.ObjectCursor@1
Right(Bob)

[sjrd]

I found why it only surfaces in scalajs-bundler! The bundle produced by scalajs-bundler doesn’t have

'use strict';

at the top of the file! It has it at the beginning of the Scala.js code, but that comes after the code inserted by the bundler for the “module system”. This means that the whole script loses strict mode, including the Scala.js-related code. If I manually add the 'use strict'; at the top of the file produced by scalajs-bundler, then it also fixes the bug!

That also explains why forcing classes fixes the issue, because the body of JavaScript classes always executes in strict mode.

It does not explain why Scala.js 1.5.0+ fixes the issue. Most likely, because for an arbitrary reason, we do not rely on having strict mode somewhere where we previously relied on it.

It does mean that the root issue is not in this repo, however, and that it can pop up again in the future, at any arbitrary time. We should fix it properly in scalajs-bundler.

[gzm0]

Thinking about this some more, I think we should at lest leave this open and ideally dedicate some time minimizing this. Especially because:

• We do not know for sure this isn’t a Scala.js issue (or at the very least I’m not convinced we know for sure based on what I read in this issue).
• If Scala.js + bundler + webpack are affected, we should care, since this isn’t a niche case.