Errors from compiler warnings don't make compilation process fail

[prolativ]

Version(s)
1.8.1

Describe the bug
Compilation errors originating from warnings do not fail the compilation process when compiling with bloop

To Reproduce

//> using scala 3.7.0
//> using options -Werror -Wunused:all

def foo =
  val x = 1
  2

scala-cli compile .

Expected behaviour
The exit code should NOT be 0.
The problem does not occur when compiling with --server=false

[Gedochao]

Huh... this can be observerd with Scala 2.13 and 2.12 (with -Xfatal-warnings) as well.

//> using scala 2.12
//> using options -Xfatal-warnings -Ywarn-unused:_

object Something {
  def foo = {
    val x = 1
    2
  }
}

and

//> using scala 2.13
//> using options -Werror -Wunused

object Something {
  def foo = {
    val x = 1
    2
  }
}

[Gedochao]

What's interesting, this still fails when we try to run... but we don't run with Bloop, we just build with it.
As far as I can tell, we're getting status code 0 from Bloop's buildTargetCompile endpoint, so Bloop claims everything is fine.

scala-cli/modules/build/src/main/scala/scala/build/Bloop.scala

Lines 53 to 57 in 815fb5d

	val compileRes = buildServer.buildTargetCompile(
	new bsp4j.CompileParams(List(buildTarget.getId).asJava)
	).get()
	val success = compileRes.getStatusCode == bsp4j.StatusCode.OK

I'd say need to investigate upstream.
cc @tgodzik

[Gedochao]

Added integration tests with the reproduction for all Scala versions in this draft PR:

• Add an integration test for whether -Werror/-Xfatal-warnings fails a compilation with Bloop properly #3736

[tgodzik]

I think the issue might be that in Bloop we remove the flag, and later turn all warnings to errors manually. This is to always get proper artefacts for Metals etc. generated.

[kubukoz]

just found this caveat 😅

Is there another way to find out if a scala-cli compile / package had warnings, other than parsing STDERR?

[Gedochao]

Is there another way to find out if a scala-cli compile / package had warnings, other than parsing STDERR?

...not really.