Project configuration fails after migrating to 0.11.2+

[MaciejG604]

After migrating from 0.11.1 to any higher version, a project, that was working fine, fails on certain commands with an exception.

It happens in this repo: https://github.com/VirtusLab/scala-js-cli e.g. after doing ./mill -i 'tests.test'

Stack trace after setting the version to 0.11.2:

Exception in thread "main" java.lang.reflect.InvocationTargetException
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:568)
        at mill.main.client.IsolatedMillMainLoader.runMain(IsolatedMillMainLoader.java:58)
        at mill.main.client.MillClientMain.main(MillClientMain.java:64)
Caused by: java.util.NoSuchElementException: head of empty list
        at scala.collection.immutable.Nil$.head(List.scala:662)
        at scala.collection.immutable.Nil$.head(List.scala:661)
        at mill.eval.GroupEvaluator$$anonfun$1.applyOrElse(GroupEvaluator.scala:103)
        at mill.eval.GroupEvaluator$$anonfun$1.applyOrElse(GroupEvaluator.scala:88)
        at scala.collection.Iterator$$anon$7.hasNext(Iterator.scala:525)
        at scala.collection.Iterator$$anon$10.hasNext(Iterator.scala:601)
        at scala.collection.IterableOnceOps.foldLeft(IterableOnce.scala:675)
        at scala.collection.IterableOnceOps.foldLeft$(IterableOnce.scala:670)
        at scala.collection.AbstractIterator.foldLeft(Iterator.scala:1300)
        at scala.collection.IterableOnceOps.sum(IterableOnce.scala:945)
        at scala.collection.IterableOnceOps.sum$(IterableOnce.scala:943)
        at scala.collection.AbstractIterator.sum(Iterator.scala:1300)
        at mill.eval.GroupEvaluator.evaluateGroupCached(GroupEvaluator.scala:136)
        at mill.eval.GroupEvaluator.evaluateGroupCached$(GroupEvaluator.scala:43)
        at mill.eval.EvaluatorImpl.evaluateGroupCached(EvaluatorImpl.scala:15)
        at mill.eval.EvaluatorCore.$anonfun$evaluate0$2(EvaluatorCore.scala:116)
        at scala.concurrent.impl.Promise$Transformation.run(Promise.scala:467)
        at mill.eval.ExecutionContexts$RunNow$.execute(ExecutionContexts.scala:13)
        at scala.concurrent.impl.Promise$Transformation.submitWithValue(Promise.scala:429)
        at scala.concurrent.impl.Promise$DefaultPromise.submitWithValue(Promise.scala:338)
        at scala.concurrent.impl.Promise$DefaultPromise.dispatchOrAddCallbacks(Promise.scala:312)
        at scala.concurrent.impl.Promise$DefaultPromise.map(Promise.scala:182)
        at mill.eval.EvaluatorCore.$anonfun$evaluate0$1(EvaluatorCore.scala:92)
        at mill.eval.EvaluatorCore.$anonfun$evaluate0$1$adapted(EvaluatorCore.scala:90)
        at scala.collection.immutable.VectorStatics$.foreachRec(Vector.scala:2122)
        at scala.collection.immutable.Vector.foreach(Vector.scala:2128)
        at mill.eval.EvaluatorCore.evaluate0(EvaluatorCore.scala:90)
        at mill.eval.EvaluatorCore.$anonfun$evaluate$1(EvaluatorCore.scala:43)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at mill.eval.EvaluatorCore.evaluate(EvaluatorCore.scala:34)
        at mill.eval.EvaluatorCore.evaluate$(EvaluatorCore.scala:26)
        at mill.eval.EvaluatorImpl.evaluate(EvaluatorImpl.scala:15)
        at mill.main.RunScript$.evaluateNamed(RunScript.scala:38)
        at mill.main.RunScript$.$anonfun$evaluateTasksNamed$2(RunScript.scala:26)
        at scala.util.Either.map(Either.scala:382)
        at mill.main.RunScript$.evaluateTasksNamed(RunScript.scala:26)
        at mill.runner.MillBuildBootstrap$.evaluateWithWatches(MillBuildBootstrap.scala:358)
        at mill.runner.MillBuildBootstrap.$anonfun$processFinalTargets$2(MillBuildBootstrap.scala:268)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at mill.runner.MillBuildBootstrap.processFinalTargets(MillBuildBootstrap.scala:268)
        at mill.runner.MillBuildBootstrap.evaluateRec(MillBuildBootstrap.scala:159)
        at mill.runner.MillBuildBootstrap.evaluate(MillBuildBootstrap.scala:47)
        at mill.runner.MillMain$.$anonfun$main0$5(MillMain.scala:207)
        at mill.runner.Watching$.watchLoop(Watching.scala:27)
        at mill.runner.MillMain$.$anonfun$main0$1(MillMain.scala:193)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at scala.Console$.withErr(Console.scala:193)
        at mill.api.SystemStreams$.$anonfun$withStreams$2(SystemStreams.scala:56)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at scala.Console$.withOut(Console.scala:164)
        at mill.api.SystemStreams$.$anonfun$withStreams$1(SystemStreams.scala:55)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at scala.Console$.withIn(Console.scala:227)
        at mill.api.SystemStreams$.withStreams(SystemStreams.scala:54)
        at mill.runner.MillMain$.main0(MillMain.scala:80)
        at mill.runner.MillMain$.main(MillMain.scala:58)
        at mill.runner.MillMain.main(MillMain.scala)
        ... 6 more

It points to main/eval/src/mill/eval/GroupEvaluator.scala line 103, where a head call has been introduced in #2417.

[lefou]

Thanks for this report. While I had a first glance, I recognized you're using some external plugins, and at least one (mill-native-image) has a problematic transitive dependency setup. This may result in an unclean classpath, as multiple versions of Mill artifacts can end up in it. This does not necessarily mean, this is the cause for the reported issue, but since Mill 0.11.2 analyzes API signatures of the whole classpath, an unclean classpath has a high potential to break this logic.

We have a new CLI option --disable-callgraph-invalidation to disable that logic, and indeed, once I use it, the reported issue goes away.

MILL_VERSION="0.11.2" mill -i --disable-callgraph-invalidation  "tests.test"
Mill version 0.11.2 is different than configured for this directory!
Configured version is 0.11.1 (/home/lefou/work/tmp/scala-js-cli/.mill-version)
[138/138] tests.test.test 
org.scalajs.cli.tests.Tests:
  + tests 7.845s

I opened alexarchambault/mill-native-image#69 for it.

[lefou]

mill-native-image version 0.1.26 was just released. @MaciejG604 Can you update to it and check, if that already fixes your issue? Thanks

[lefou]

Bumping mill-native-image does not fix the issue.

diff --git a/build.sc b/build.sc
index 54f7eda..57610af 100644
--- a/build.sc
+++ b/build.sc
@@ -1,6 +1,6 @@
 import $ivy.`de.tototec::de.tobiasroeser.mill.vcs.version::0.4.0`
-import $ivy.`io.github.alexarchambault.mill::mill-native-image::0.1.25`
-import $ivy.`io.github.alexarchambault.mill::mill-native-image-upload:0.1.19`
+import $ivy.`io.github.alexarchambault.mill::mill-native-image::0.1.26`
+import $ivy.`io.github.alexarchambault.mill::mill-native-image-upload:0.1.26`
 import $ivy.`io.get-coursier::coursier-launcher:2.1.0-M2`
 
 import de.tobiasroeser.mill.vcs.version._
@@ -112,7 +112,7 @@ trait ScalaJsCliNativeImage extends ScalaModule with NativeImage {
   def graalVmVersion = "22.3.3"
   def nativeImageGraalVmJvmId = s"graalvm-java17:$graalVmVersion"
   def nativeImageName = "scala-js-ld"
-  def moduleDeps() = Seq(
+  def moduleDeps = Seq(
     cli
   )
   def compileIvyDeps = super.compileIvyDeps() ++ Seq(
@@ -353,7 +353,7 @@ object ci extends Module {
 
     val path = os.Path(directory, os.pwd)
     val launchers = os.list(path).filter(os.isFile(_)).map { path =>
-      path.toNIO -> path.last
+      path -> path.last
     }
     val ghToken = Option(System.getenv("UPLOAD_GH_TOKEN")).getOrElse {
       sys.error("UPLOAD_GH_TOKEN not set")

Output:

$ ± MILL_VERSION="0.11.5" mill -i "tests.test"
Mill version 0.11.5 is different than configured for this directory!
Configured version is 0.11.1 (/home/lefou/work/tmp/scala-js-cli/.mill-version)
[130/138] tests.test.forkEnv 
An unexpected error occurred
Exception in thread "main" java.lang.reflect.InvocationTargetException
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:566)
        at mill.main.client.IsolatedMillMainLoader.runMain(IsolatedMillMainLoader.java:58)
        at mill.main.client.MillClientMain.main(MillClientMain.java:64)
Caused by: java.util.NoSuchElementException: head of empty list
        at scala.collection.immutable.Nil$.head(List.scala:662)
        at scala.collection.immutable.Nil$.head(List.scala:661)
        at mill.eval.GroupEvaluator$$anonfun$1.applyOrElse(GroupEvaluator.scala:104)
        at mill.eval.GroupEvaluator$$anonfun$1.applyOrElse(GroupEvaluator.scala:89)
        at scala.collection.Iterator$$anon$7.hasNext(Iterator.scala:525)
        at scala.collection.Iterator$$anon$10.hasNext(Iterator.scala:601)
        at scala.collection.IterableOnceOps.foldLeft(IterableOnce.scala:675)
        at scala.collection.IterableOnceOps.foldLeft$(IterableOnce.scala:670)
        at scala.collection.AbstractIterator.foldLeft(Iterator.scala:1300)
        at scala.collection.IterableOnceOps.sum(IterableOnce.scala:945)
        at scala.collection.IterableOnceOps.sum$(IterableOnce.scala:943)
        at scala.collection.AbstractIterator.sum(Iterator.scala:1300)
        at mill.eval.GroupEvaluator.evaluateGroupCached(GroupEvaluator.scala:137)
        at mill.eval.GroupEvaluator.evaluateGroupCached$(GroupEvaluator.scala:44)
        at mill.eval.EvaluatorImpl.evaluateGroupCached(EvaluatorImpl.scala:15)
        at mill.eval.EvaluatorCore.$anonfun$evaluate0$2(EvaluatorCore.scala:116)
        at scala.concurrent.impl.Promise$Transformation.run(Promise.scala:467)
        at mill.eval.ExecutionContexts$RunNow$.execute(ExecutionContexts.scala:13)
        at scala.concurrent.impl.Promise$Transformation.submitWithValue(Promise.scala:429)
        at scala.concurrent.impl.Promise$DefaultPromise.submitWithValue(Promise.scala:338)
        at scala.concurrent.impl.Promise$DefaultPromise.dispatchOrAddCallbacks(Promise.scala:312)
        at scala.concurrent.impl.Promise$DefaultPromise.map(Promise.scala:182)
        at mill.eval.EvaluatorCore.$anonfun$evaluate0$1(EvaluatorCore.scala:92)
        at mill.eval.EvaluatorCore.$anonfun$evaluate0$1$adapted(EvaluatorCore.scala:90)
        at scala.collection.immutable.VectorStatics$.foreachRec(Vector.scala:2124)
        at scala.collection.immutable.Vector.foreach(Vector.scala:2130)
        at mill.eval.EvaluatorCore.evaluate0(EvaluatorCore.scala:90)
        at mill.eval.EvaluatorCore.$anonfun$evaluate$1(EvaluatorCore.scala:43)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at mill.eval.EvaluatorCore.evaluate(EvaluatorCore.scala:34)
        at mill.eval.EvaluatorCore.evaluate$(EvaluatorCore.scala:26)
        at mill.eval.EvaluatorImpl.evaluate(EvaluatorImpl.scala:15)
        at mill.main.RunScript$.evaluateNamed(RunScript.scala:38)
        at mill.main.RunScript$.$anonfun$evaluateTasksNamed$2(RunScript.scala:26)
        at scala.util.Either.map(Either.scala:382)
        at mill.main.RunScript$.evaluateTasksNamed(RunScript.scala:26)
        at mill.runner.MillBuildBootstrap$.evaluateWithWatches(MillBuildBootstrap.scala:399)
        at mill.runner.MillBuildBootstrap.$anonfun$processFinalTargets$3(MillBuildBootstrap.scala:308)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at mill.runner.MillBuildBootstrap.processFinalTargets(MillBuildBootstrap.scala:308)
        at mill.runner.MillBuildBootstrap.evaluateRec(MillBuildBootstrap.scala:196)
        at mill.runner.MillBuildBootstrap.evaluate(MillBuildBootstrap.scala:48)
        at mill.runner.MillMain$.$anonfun$main0$6(MillMain.scala:234)
        at mill.runner.Watching$.watchLoop(Watching.scala:27)
        at mill.runner.MillMain$.$anonfun$main0$1(MillMain.scala:219)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at scala.Console$.withErr(Console.scala:193)
        at mill.api.SystemStreams$.$anonfun$withStreams$2(SystemStreams.scala:62)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at scala.Console$.withOut(Console.scala:164)
        at mill.api.SystemStreams$.$anonfun$withStreams$1(SystemStreams.scala:61)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at scala.Console$.withIn(Console.scala:227)
        at mill.api.SystemStreams$.withStreams(SystemStreams.scala:60)
        at mill.runner.MillMain$.main0(MillMain.scala:101)
        at mill.runner.MillMain$.liftedTree1$1(MillMain.scala:78)
        at mill.runner.MillMain$.main(MillMain.scala:69)
        at mill.runner.MillMain.main(MillMain.scala)
        ... 6 more

[lefou]

@lihaoyi Could you have a look?

[Gedochao]

@lefou although --disable-callgraph-invalidation seems to solve the core issue, we ended up reverting to Mill 0.11.1, as the workaround seems to have broken our CI.

I did try to work around it in VirtusLab/scala-cli#2528, but eventually gave up with yet another failure (unclear if because of 0.11.5, --disable-callgraph-invalidation or some other problem with the Scala CLI build configuration). There was definitely a big chunk of headaches tied to #2869 sneakily breaking our publishing.

Looking forward to this issue getting fixed, regardless, we'll stay on 0.11.1 until then.

[lefou]

I had some spare time and invested in narrowing down this issue. First, I made sure this is not related to external dependencies. I found an packaging issue in mill-native-image which is fixed in the meantime. So I opened PR VirtusLab/scala-js-cli#38.

I also found some other issues which I addressed with PR VirtusLab/scala-js-cli#39.

So, based on the branch/PR VirtusLab/scala-js-cli#39, I'm able to reproduce or mute the reported issue with a change of a single line:

The failure with Mill 0.11.5

diff --git a/.mill-version b/.mill-version
index 027934e..ce3191f 100644
--- a/.mill-version
+++ b/.mill-version
@@ -1 +1 @@
-0.11.1
\ No newline at end of file
+0.11.5-29-bc84d5

> mill -i -w tests.test
[134/141] tests.test.forkWorkingDir 
An unexpected error occurred
Exception in thread "main" java.lang.reflect.InvocationTargetException
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:568)
        at mill.main.client.IsolatedMillMainLoader.runMain(IsolatedMillMainLoader.java:58)
        at mill.main.client.MillClientMain.main(MillClientMain.java:64)
Caused by: java.util.NoSuchElementException: head of empty list
        at scala.collection.immutable.Nil$.head(List.scala:662)
        at scala.collection.immutable.Nil$.head(List.scala:661)
        at mill.eval.GroupEvaluator$$anonfun$1.applyOrElse(GroupEvaluator.scala:104)
        at mill.eval.GroupEvaluator$$anonfun$1.applyOrElse(GroupEvaluator.scala:89)
        at scala.collection.Iterator$$anon$7.hasNext(Iterator.scala:525)
        at scala.collection.Iterator$$anon$10.hasNext(Iterator.scala:601)
        at scala.collection.IterableOnceOps.foldLeft(IterableOnce.scala:675)
        at scala.collection.IterableOnceOps.foldLeft$(IterableOnce.scala:670)
        at scala.collection.AbstractIterator.foldLeft(Iterator.scala:1300)
        at scala.collection.IterableOnceOps.sum(IterableOnce.scala:945)
        at scala.collection.IterableOnceOps.sum$(IterableOnce.scala:943)
        at scala.collection.AbstractIterator.sum(Iterator.scala:1300)
        at mill.eval.GroupEvaluator.evaluateGroupCached(GroupEvaluator.scala:137)
        at mill.eval.GroupEvaluator.evaluateGroupCached$(GroupEvaluator.scala:44)
        at mill.eval.EvaluatorImpl.evaluateGroupCached(EvaluatorImpl.scala:15)
        at mill.eval.EvaluatorCore.$anonfun$evaluate0$2(EvaluatorCore.scala:116)
        at scala.concurrent.impl.Promise$Transformation.run(Promise.scala:467)
        at mill.eval.ExecutionContexts$RunNow$.execute(ExecutionContexts.scala:13)
        at scala.concurrent.impl.Promise$Transformation.submitWithValue(Promise.scala:429)
        at scala.concurrent.impl.Promise$DefaultPromise.submitWithValue(Promise.scala:338)
        at scala.concurrent.impl.Promise$DefaultPromise.dispatchOrAddCallbacks(Promise.scala:312)
        at scala.concurrent.impl.Promise$DefaultPromise.map(Promise.scala:182)
        at mill.eval.EvaluatorCore.$anonfun$evaluate0$1(EvaluatorCore.scala:92)
        at mill.eval.EvaluatorCore.$anonfun$evaluate0$1$adapted(EvaluatorCore.scala:90)
        at scala.collection.immutable.Vector.foreach(Vector.scala:2124)
        at mill.eval.EvaluatorCore.evaluate0(EvaluatorCore.scala:90)
        at mill.eval.EvaluatorCore.$anonfun$evaluate$1(EvaluatorCore.scala:43)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at mill.eval.EvaluatorCore.evaluate(EvaluatorCore.scala:34)
        at mill.eval.EvaluatorCore.evaluate$(EvaluatorCore.scala:26)
        at mill.eval.EvaluatorImpl.evaluate(EvaluatorImpl.scala:15)
        at mill.main.RunScript$.evaluateNamed(RunScript.scala:38)
        at mill.main.RunScript$.$anonfun$evaluateTasksNamed$2(RunScript.scala:26)
        at scala.util.Either.map(Either.scala:382)
        at mill.main.RunScript$.evaluateTasksNamed(RunScript.scala:26)
        at mill.runner.MillBuildBootstrap$.evaluateWithWatches(MillBuildBootstrap.scala:399)
        at mill.runner.MillBuildBootstrap.$anonfun$processFinalTargets$3(MillBuildBootstrap.scala:308)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at mill.runner.MillBuildBootstrap.processFinalTargets(MillBuildBootstrap.scala:308)
        at mill.runner.MillBuildBootstrap.evaluateRec(MillBuildBootstrap.scala:196)
        at mill.runner.MillBuildBootstrap.evaluate(MillBuildBootstrap.scala:48)
        at mill.runner.MillMain$.$anonfun$main0$6(MillMain.scala:234)
        at mill.runner.Watching$.watchLoop(Watching.scala:27)
        at mill.runner.MillMain$.$anonfun$main0$1(MillMain.scala:219)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at scala.Console$.withErr(Console.scala:193)
        at mill.api.SystemStreams$.$anonfun$withStreams$2(SystemStreams.scala:62)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at scala.Console$.withOut(Console.scala:164)
        at mill.api.SystemStreams$.$anonfun$withStreams$1(SystemStreams.scala:61)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at scala.Console$.withIn(Console.scala:227)
        at mill.api.SystemStreams$.withStreams(SystemStreams.scala:60)
        at mill.runner.MillMain$.main0(MillMain.scala:101)
        at mill.runner.MillMain$.liftedTree1$1(MillMain.scala:78)
        at mill.runner.MillMain$.main(MillMain.scala:69)
        at mill.runner.MillMain.main(MillMain.scala)
        ... 6 more

The single-line change to remove the reported error

diff --git a/.mill-version b/.mill-version
index 027934e..ce3191f 100644
--- a/.mill-version
+++ b/.mill-version
@@ -1 +1 @@
-0.11.1
\ No newline at end of file
+0.11.5-29-bc84d5
diff --git a/build.sc b/build.sc
index 9df587f..1c7c722 100644
--- a/build.sc
+++ b/build.sc
@@ -55,7 +55,7 @@ trait Cli extends ScalaModule with ScalaJsCliPublishModule {
       Parameters,
       Preamble
     }
-    val cp = jarClassPath().map(_.path)
+    val cp = Seq.empty[os.Path]
     val mainClass0 = mainClass().getOrElse(sys.error("No main class"))
 
     val dest = T.ctx().dest / (if (isWin) "launcher.bat" else "launcher")

> mill -i -w tests.test
[build.sc] [49/53] compile 
[info] compiling 1 Scala source to /home/lefou/work/tmp/scala-js-cli/out/mill-build/compile.dest/classes ...
[warn] /home/lefou/work/tmp/scala-js-cli/build.sc:315:21: constructor SonatypePublisher in class SonatypePublisher is deprecated (since mill 0.10.8): Use other constructor instead
[warn]     val publisher = new scalalib.publish.SonatypePublisher(
[warn]                     ^
[warn] one warning found
[info] done compiling
[88/88] tests.test.jvm 
org.scalajs.cli.tests.Tests:
==> X org.scalajs.cli.tests.Tests.tests  3.129s os.SubprocessException: Result of /home/lefou/work/tmp/scala-js-cli/out/cli/standaloneLauncher.dest/launcher…: 255
Error: class org.scalajs.cli.Scalajsld not found

    at os.proc.call(ProcessOps.scala:89)
    at org.scalajs.cli.tests.Tests.$anonfun$new$1(Tests.scala:48)
1 targets failed
tests.test.jvm 1 tests failed: 
  org.scalajs.cli.tests.Tests.tests org.scalajs.cli.tests.Tests.tests

Obviously the test fails, as I have just emptied the jarClasspath, but the issue in Mill is gone. I hope this helps @lihaoyi to further analyze the classpath analyzer?

[lefou]

Here is a minimal reproducer:

// /tmp/mill-2844/build.sc
import mill._

object foo extends Module {
  def bar = T {
    baz()
  }
}

private def baz = T { "baz" }

> mill -i dev.run /tmp/mill-2844 -i foo.bar
[1691/1691] dev.run 
[info] compiling 1 Scala source to /tmp/mill-2844/out/mill-build/compile.dest/classes ...
[info] done compiling
An unexpected error occurred
Exception in thread "main" java.lang.reflect.InvocationTargetException
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:568)
        at mill.main.client.IsolatedMillMainLoader.runMain(IsolatedMillMainLoader.java:58)
        at mill.main.client.MillClientMain.main(MillClientMain.java:64)
Caused by: java.util.NoSuchElementException: head of empty list
        at scala.collection.immutable.Nil$.head(List.scala:662)
        at scala.collection.immutable.Nil$.head(List.scala:661)
        at mill.eval.GroupEvaluator$$anonfun$1.applyOrElse(GroupEvaluator.scala:104)
        at mill.eval.GroupEvaluator$$anonfun$1.applyOrElse(GroupEvaluator.scala:89)
        at scala.collection.Iterator$$anon$7.hasNext(Iterator.scala:525)
        at scala.collection.Iterator$$anon$10.hasNext(Iterator.scala:601)
        at scala.collection.IterableOnceOps.foldLeft(IterableOnce.scala:675)
        at scala.collection.IterableOnceOps.foldLeft$(IterableOnce.scala:670)
        at scala.collection.AbstractIterator.foldLeft(Iterator.scala:1300)
        at scala.collection.IterableOnceOps.sum(IterableOnce.scala:945)
        at scala.collection.IterableOnceOps.sum$(IterableOnce.scala:943)
        at scala.collection.AbstractIterator.sum(Iterator.scala:1300)
        at mill.eval.GroupEvaluator.evaluateGroupCached(GroupEvaluator.scala:137)
        at mill.eval.GroupEvaluator.evaluateGroupCached$(GroupEvaluator.scala:44)
        at mill.eval.EvaluatorImpl.evaluateGroupCached(EvaluatorImpl.scala:15)
        at mill.eval.EvaluatorCore.$anonfun$evaluate0$2(EvaluatorCore.scala:116)
        at scala.concurrent.impl.Promise$Transformation.run(Promise.scala:467)
        at mill.eval.ExecutionContexts$RunNow$.execute(ExecutionContexts.scala:13)
        at scala.concurrent.impl.Promise$Transformation.submitWithValue(Promise.scala:429)
        at scala.concurrent.impl.Promise$DefaultPromise.submitWithValue(Promise.scala:338)
        at scala.concurrent.impl.Promise$DefaultPromise.dispatchOrAddCallbacks(Promise.scala:312)
        at scala.concurrent.impl.Promise$DefaultPromise.map(Promise.scala:182)
        at mill.eval.EvaluatorCore.$anonfun$evaluate0$1(EvaluatorCore.scala:92)
        at mill.eval.EvaluatorCore.$anonfun$evaluate0$1$adapted(EvaluatorCore.scala:90)
        at scala.collection.immutable.Vector.foreach(Vector.scala:2124)
        at mill.eval.EvaluatorCore.evaluate0(EvaluatorCore.scala:90)
        at mill.eval.EvaluatorCore.$anonfun$evaluate$1(EvaluatorCore.scala:43)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at mill.eval.EvaluatorCore.evaluate(EvaluatorCore.scala:34)
        at mill.eval.EvaluatorCore.evaluate$(EvaluatorCore.scala:26)
        at mill.eval.EvaluatorImpl.evaluate(EvaluatorImpl.scala:15)
        at mill.main.RunScript$.evaluateNamed(RunScript.scala:38)
        at mill.main.RunScript$.$anonfun$evaluateTasksNamed$2(RunScript.scala:26)
        at scala.util.Either.map(Either.scala:382)
        at mill.main.RunScript$.evaluateTasksNamed(RunScript.scala:26)
        at mill.runner.MillBuildBootstrap$.evaluateWithWatches(MillBuildBootstrap.scala:399)
        at mill.runner.MillBuildBootstrap.$anonfun$processFinalTargets$3(MillBuildBootstrap.scala:308)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at mill.runner.MillBuildBootstrap.processFinalTargets(MillBuildBootstrap.scala:308)
        at mill.runner.MillBuildBootstrap.evaluateRec(MillBuildBootstrap.scala:196)
        at mill.runner.MillBuildBootstrap.evaluate(MillBuildBootstrap.scala:48)
        at mill.runner.MillMain$.$anonfun$main0$6(MillMain.scala:234)
        at mill.runner.Watching$.watchLoop(Watching.scala:27)
        at mill.runner.MillMain$.$anonfun$main0$1(MillMain.scala:219)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at scala.Console$.withErr(Console.scala:193)
        at mill.api.SystemStreams$.$anonfun$withStreams$2(SystemStreams.scala:62)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at scala.Console$.withOut(Console.scala:164)
        at mill.api.SystemStreams$.$anonfun$withStreams$1(SystemStreams.scala:61)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
        at scala.Console$.withIn(Console.scala:227)
        at mill.api.SystemStreams$.withStreams(SystemStreams.scala:60)
        at mill.runner.MillMain$.main0(MillMain.scala:101)
        at mill.runner.MillMain$.liftedTree1$1(MillMain.scala:78)
        at mill.runner.MillMain$.main(MillMain.scala:69)
        at mill.runner.MillMain.main(MillMain.scala)
        ... 6 more

Removing private from baz fixes it.

> mill -i dev.run /tmp/mill-2844 -i show foo.bar
[1691/1691] dev.run 
[info] compiling 1 Scala source to /tmp/mill-2844/out/mill-build/compile.dest/classes ...
[info] done compiling
"baz"