mill _.repl fails if module depends on mainargs

[KireinaHoro]

It seems like _.repl fails to launch when the module depends on mainargs as well. Reproducible case:

import mill._
import mill.util._
import scalalib._

object mymod extends ScalaModule {
  def scalaVersion = "2.13.12"
  override def ivyDeps = Agg(
    ivy"com.lihaoyi::mainargs:0.5.4"
  )
}

Output:

-> % mill mymod.repl
No mill version specified.
You should provide a version via '.mill-version' file or --mill-version option.
Using mill version 0.11.6
[build.sc] [49/53] compile
[info] compiling 1 Scala source to /tmp/out/mill-build/compile.dest/classes ...
[info] done compiling
[46/46] mymod.repl
Exception in thread "main" java.lang.NoClassDefFoundError: mainargs/ArgReader
        at ammonite.AmmoniteMain$.main0(AmmoniteMain.scala:51)
        at ammonite.AmmoniteMain$.main(AmmoniteMain.scala:31)
        at ammonite.AmmoniteMain.main(AmmoniteMain.scala)
Caused by: java.lang.ClassNotFoundException: mainargs.ArgReader
        at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:581)
        at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
        at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:527)
        ... 3 more
1 targets failed
mymod.repl java.lang.Exception: Interactive Subprocess Failed (exit code 1)
    mill.util.Jvm$.runSubprocess(Jvm.scala:184)
    mill.util.Jvm$.runSubprocessWithBackgroundOutputs(Jvm.scala:152)
    mill.util.Jvm$.runSubprocess(Jvm.scala:88)
    mill.scalalib.ScalaModule.$anonfun$repl$2(ScalaModule.scala:441)
    scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
    scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
    scala.Console$.withErr(Console.scala:193)
    mill.api.SystemStreams$.$anonfun$withStreams$2(SystemStreams.scala:62)
    scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
    scala.Console$.withOut(Console.scala:164)
    mill.api.SystemStreams$.$anonfun$withStreams$1(SystemStreams.scala:61)
    scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
    scala.Console$.withIn(Console.scala:227)
    mill.api.SystemStreams$.withStreams(SystemStreams.scala:60)
    mill.scalalib.ScalaModule.$anonfun$repl$1(ScalaModule.scala:448)
    mill.define.Task$TraverseCtx.evaluate(Task.scala:71)

[lefou]

This concrete issue is likely fixed, since we updated mainargs to it's latest version in PR #3019.

But this does not fix the general issue completely. One aspect is the version pinning we could improve (we already working towards that), the other aspect is that there is always a slight chance the repl will fail, when your local project / classpath has conflicting dependencies with the repl itself. A workaround in such situations can be to use the alternative console, which might have less features, but also less dependencies.