`abstract case class` breaks Scala.js sourcemaps?

[armanbilge]

(originally reported by @mn98 as a possible Cats Effect bug)

Compiler version

3.1.1-RC2

Minimized code

// build.sbt
scalaVersion := "3.1.1-RC2"
scalaJSUseMainModuleInitializer := true
enablePlugins(ScalaJSPlugin, ScalaJSBundlerPlugin)

// project/plugins.sbt
addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.8.0")
addSbtPlugin("ch.epfl.scala" % "sbt-scalajs-bundler" % "0.20.0")

// src/main/scala/Main.scala
object Main {
  def main(args: Array[String]): Unit = ExitCode(0)
}
abstract case class ExitCode private (code: Int)
object ExitCode {
  def apply(i: Int): ExitCode = new ExitCode(i & 0xff) {}
  val Success: ExitCode = ExitCode(0)
}

Output

sbt> Compile / fastOptJS / webpack
[info] compiling 1 Scala source to /workspace/http4s4tw/target/scala-3.1.1-RC2/classes ...
[info] Updating NPM dependencies
[info] Fast optimizing /workspace/http4s4tw/target/scala-3.1.1-RC2/http4s4tw-fastopt
[error] npm WARN deprecated urix@0.1.0: Please see https://github.com/lydell/urix#deprecated
[error] npm WARN deprecated resolve-url@0.2.1: https://github.com/lydell/resolve-url#deprecated
[error] npm WARN deprecated chokidar@2.1.8: Chokidar 2 will break on node v14+. Upgrade to chokidar 3 with 15x less dependencies.
[error] npm WARN deprecated chokidar@2.1.8: Chokidar 2 will break on node v14+. Upgrade to chokidar 3 with 15x less dependencies.
[error] npm WARN deprecated querystring@0.2.0: The querystring API is considered Legacy. new code should use the URLSearchParams API instead.
[error] npm WARN deprecated uuid@3.4.0: Please upgrade  to version 7 or higher.  Older versions may use Math.random() in certain circumstances, which is known to be problematic.  See https://v8.dev/blog/math-random for details.
[info] added 603 packages, and audited 604 packages in 10s
[info] 20 packages are looking for funding
[info]   run `npm fund` for details
[info] 14 vulnerabilities (8 moderate, 6 high)
[info] To address issues that do not require attention, run:
[info]   npm audit fix
[info] To address all issues, run:
[info]   npm audit fix --force
[info] Run `npm audit` for details.
[info] Writing scalajs.webpack.config.js
[info] Bundling the application with its NPM dependencies
[error] /workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/source-map/lib/source-map-generator.js:298
[error]       throw new Error('Invalid mapping: ' + JSON.stringify({
[error]       ^
[error] Error: Invalid mapping: {"generated":{"line":969,"column":24},"source":"/workspace/http4s4tw/src/main/scala/Main.scala","original":{"line":4,"column":-1},"name":"toString"}
[error]     at SourceMapGenerator_validateMapping [as _validateMapping] (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/source-map/lib/source-map-generator.js:298:13)
[error]     at SourceMapGenerator_addMapping [as addMapping] (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/source-map/lib/source-map-generator.js:110:12)
[error]     at /workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/source-map/lib/source-node.js:351:13
[error]     at SourceNode_walk [as walk] (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/source-map/lib/source-node.js:230:9)
[error]     at SourceNode_walk [as walk] (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/source-map/lib/source-node.js:226:13)
[error]     at SourceNode_walk [as walk] (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/source-map/lib/source-node.js:226:13)
[error]     at SourceNode_toStringWithSourceMap [as toStringWithSourceMap] (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/source-map/lib/source-node.js:342:8)
[error]     at ConcatSource.proto.sourceAndMap (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/webpack-sources/lib/SourceAndMapMixin.js:29:32)
[error]     at CachedSource.sourceAndMap (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/webpack-sources/lib/CachedSource.js:58:31)
[error]     at getTaskForFile (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/webpack/lib/SourceMapDevToolPlugin.js:37:30)
[error]     at /workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/webpack/lib/SourceMapDevToolPlugin.js:136:20
[error]     at Array.forEach (<anonymous>)
[error]     at /workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/webpack/lib/SourceMapDevToolPlugin.js:130:12
[error]     at SyncHook.eval [as call] (eval at create (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/tapable/lib/HookCodeFactory.js:19:10), <anonymous>:7:1)
[error]     at SyncHook.lazyCompileHook (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/tapable/lib/Hook.js:154:20)
[error]     at /workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/webpack/lib/Compilation.js:1319:42
[error]     at AsyncSeriesHook.eval [as callAsync] (eval at create (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:6:1)
[error]     at AsyncSeriesHook.lazyCompileHook (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/tapable/lib/Hook.js:154:20)
[error]     at /workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/webpack/lib/Compilation.js:1315:36
[error]     at AsyncSeriesHook.eval [as callAsync] (eval at create (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:6:1)
[error]     at AsyncSeriesHook.lazyCompileHook (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/tapable/lib/Hook.js:154:20)
[error]     at /workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/webpack/lib/Compilation.js:1311:32
[error]     at AsyncSeriesHook.eval [as callAsync] (eval at create (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:6:1)
[error]     at AsyncSeriesHook.lazyCompileHook (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/tapable/lib/Hook.js:154:20)
[error]     at Compilation.seal (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/webpack/lib/Compilation.js:1248:27)
[error]     at /workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/webpack/lib/Compiler.js:625:18
[error]     at /workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/webpack/lib/Compilation.js:1171:4
[error]     at AsyncSeriesHook.eval [as callAsync] (eval at create (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:15:1)
[error]     at AsyncSeriesHook.lazyCompileHook (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/tapable/lib/Hook.js:154:20)
[error]     at Compilation.finish (/workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/webpack/lib/Compilation.js:1163:28)
[error] Failure on parsing the output of webpack: No content to map due to end-of-input
[error]  at [Source: java.lang.ProcessImpl$ProcessPipeInputStream@3fa6bef7; line: 1, column: 0]
[error] You can try to manually execute the command
[error] node /workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/node_modules/webpack/bin/webpack --bail --profile --json --config /workspace/http4s4tw/target/scala-3.1.1-RC2/scalajs-bundler/main/scalajs.webpack.config.js
[error] 
[error] stack trace is suppressed; run last Compile / fastOptJS for the full output
[error] (Compile / fastOptJS / webpack) Non-zero exit code: 1
[error] Total time: 12 s, completed Jan 10, 2022, 4:01:39 PM

Expectation

Switching to Scala 2.13.7 there is no error.

[armanbilge]

If it's helpful, here's how I was able to re-implement ExitCode to work around this issue.

• Workaround for dotty#14240 typelevel/cats-effect#2743

[SethTisue]

Is this truly minimized, is all of the code necessary? Or is it just any abstract case class?

[SethTisue]

Is there reason to believe the bug is in this repo and not in the scalajs-bundler repo?

[armanbilge]

Is this truly minimized, is all of the code necessary?

It can possibly be slightly more minimized. Like the Int mask thing, woops. But the val in the companion object was also necessary. Edit: oh, and the private constructor also seems entangled.

Is there reason to believe the bug is in this repo and not in the scalars-bundler repo?

Not directly. The same code does work with Scala 2.x, but I can't rule out the possibility that Scala 3.x is generating different but equally valid output that the bundling thing doesn't understand.

I don't understand the Scala.js pipeline well enough to know when/how sourcemaps get generated, but another experiment I did was using the Scala 2.13 artifacts for Cats Effect in a Scala 3.1 project, and I had no problems there. So whatever is different I assume is in the Scala 3 sjsir artifacts for Cats Effect.

[SethTisue]

Okay. Let's wait for Scala.js expert to weigh in on where we should look next.

[sjrd]

The column:-1 in "original":{"line":4,"column":-1} is suspicious. All line and column numbers should be >= 0.

Scala 2 has 1-based positions, but the IR has 0-based position, so we do {line,column} - 1 when translating the scalac Positions to the IR Positions. We do the same in Dotty, but could it be that Dotty has 0-based positions?

---

Edit: Oops, yes, looks like it does:
https://github.com/lampepfl/dotty/blob/c49a1cf106f2f4df06b8b06d94fcf76d687b1bdb/compiler/src/dotty/tools/dotc/util/SourceFile.scala#L174-L177
https://github.com/lampepfl/dotty/blob/c49a1cf106f2f4df06b8b06d94fcf76d687b1bdb/compiler/src/dotty/tools/dotc/util/SourceFile.scala#L197-L198

[armanbilge]

@sjrd Thanks for tracking this down! A question: assuming (IIUC) that this bug has affected all currently published Scala 3 SJSIR artifacts, it (mildly) breaks the promise undoes the advantage of backwards-compatibility since everyone would have to republish. Is there any way for the Scala.js linker to detect it's consuming IR generated by a pre-fix Scala 3 and apply an off-by-one correction? Thanks!