Before:
```
[warn] /Users/jz/code/zinc/internal/zinc-apiinfo/src/main/scala/xsbt/api/Visit.scala:187:19: parameter value s in method visitString is never used
[warn] def visitString(s: String): Unit = ()
[warn] ^
^C
[warn] Canceling execution...
[warn] 10 warnings found
[info] Compilation has been cancelled
[error] java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.ExecutorCompletionService$QueueingFuture@33ec70dd rejected from java.util.concurrent.ThreadPoolExecutor@4b832de6[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 1410]
[error] at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2063)
[error] at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:830)
[error] at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1379)
[error] at java.util.concurrent.ExecutorCompletionService.submit(ExecutorCompletionService.java:181)
[error] at sbt.CompletionService$.submit(CompletionService.scala:36)
[error] at sbt.ConcurrentRestrictions$$anon$4.submitValid(ConcurrentRestrictions.scala:176)
[error] at sbt.ConcurrentRestrictions$$anon$4.submit(ConcurrentRestrictions.scala:165)
[error] at sbt.Execute.submit(Execute.scala:262)
[error] at sbt.Execute.ready(Execute.scala:242)
[error] at sbt.Execute.notifyDone(Execute.scala:181)
[error] at sbt.Execute.$anonfun$retire$2(Execute.scala:152)
[error] at sbt.Execute.$anonfun$retire$2$adapted(Execute.scala:151)
[error] at scala.collection.immutable.List.foreach(List.scala:389)
[error] at sbt.Execute.retire(Execute.scala:151)
[error] at sbt.Execute.$anonfun$work$2(Execute.scala:279)
[error] at sbt.Execute$$anon$1.process(Execute.scala:24)
[error] at sbt.Execute.next$1(Execute.scala:104)
[error] at sbt.Execute.processAll(Execute.scala:107)
[error] at sbt.Execute.runKeep(Execute.scala:84)
[error] at sbt.EvaluateTask$.liftedTree1$1(EvaluateTask.scala:387)
[error] at sbt.EvaluateTask$.run$1(EvaluateTask.scala:386)
[error] at sbt.EvaluateTask$.runTask(EvaluateTask.scala:405)
[error] at sbt.internal.Aggregation$.$anonfun$timedRun$4(Aggregation.scala:100)
[error] at sbt.EvaluateTask$.withStreams(EvaluateTask.scala:331)
[error] at sbt.internal.Aggregation$.timedRun(Aggregation.scala:98)
[error] at sbt.internal.Aggregation$.runTasks(Aggregation.scala:111)
[error] at sbt.internal.Aggregation$.$anonfun$applyTasks$1(Aggregation.scala:68)
[error] at sbt.Command$.$anonfun$applyEffect$2(Command.scala:130)
[error] at sbt.internal.Aggregation$.$anonfun$evaluatingParser$11(Aggregation.scala:220)
[error] at sbt.internal.Act$.$anonfun$actParser0$3(Act.scala:387)
[error] at sbt.MainLoop$.processCommand(MainLoop.scala:154)
[error] at sbt.MainLoop$.$anonfun$next$2(MainLoop.scala:137)
[error] at sbt.State$$anon$1.runCmd$1(State.scala:242)
[error] at sbt.State$$anon$1.process(State.scala:248)
[error] at sbt.MainLoop$.$anonfun$next$1(MainLoop.scala:137)
[error] at sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:16)
[error] at sbt.MainLoop$.next(MainLoop.scala:137)
[error] at sbt.MainLoop$.run(MainLoop.scala:130)
[error] at sbt.MainLoop$.$anonfun$runWithNewLog$1(MainLoop.scala:108)
[error] at sbt.io.Using.apply(Using.scala:22)
[error] at sbt.MainLoop$.runWithNewLog(MainLoop.scala:102)
[error] at sbt.MainLoop$.runAndClearLast(MainLoop.scala:58)
[error] at sbt.MainLoop$.runLoggedLoop(MainLoop.scala:43)
[error] at sbt.MainLoop$.runLogged(MainLoop.scala:35)
[error] at sbt.StandardMain$.runManaged(Main.scala:113)
[error] at sbt.xMain.run(Main.scala:76)
[error] at xsbt.boot.Launch$$anonfun$run$1.apply(Launch.scala:109)
[error] at xsbt.boot.Launch$.withContextLoader(Launch.scala:128)
[error] at xsbt.boot.Launch$.run(Launch.scala:109)
[error] at xsbt.boot.Launch$$anonfun$apply$1.apply(Launch.scala:35)
[error] at xsbt.boot.Launch$.launch(Launch.scala:117)
[error] at xsbt.boot.Launch$.apply(Launch.scala:18)
[error] at xsbt.boot.Boot$.runImpl(Boot.scala:41)
[error] at xsbt.boot.Boot$.main(Boot.scala:17)
[error] at xsbt.boot.Boot.main(Boot.scala)
[error] java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.ExecutorCompletionService$QueueingFuture@33ec70dd rejected from java.util.concurrent.ThreadPoolExecutor@4b832de6[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 1410]
[error] Use 'last' for the full log.
```
After:
```
sbt:zinc Root> ;zincIvyIntegration/cleanClasses;zincIvyIntegration/compile
[success] Total time: 0 s, completed 03/04/2018 2:24:33 PM
[info] Compiling 5 Scala sources and 1 Java source to /Users/jz/code/zinc/internal/zinc-ivy-integration/target/scala-2.12/classes ...
[warn] /Users/jz/code/zinc/internal/zinc-ivy-integration/src/main/scala/sbt/internal/inc/ResourceLoader.scala:14:42: parameter value classLoader in method getPropertiesFor is never used
[warn] def getPropertiesFor(resource: String, classLoader: ClassLoader): Properties = {
[warn] ^
[warn] /Users/jz/code/zinc/internal/zinc-ivy-integration/src/main/scala/sbt/internal/inc/ZincComponentCompiler.scala:13:17: Unused import
[warn] import java.net.URLClassLoader
[warn] ^
[warn] /Users/jz/code/zinc/internal/zinc-ivy-integration/src/main/scala/sbt/internal/inc/ZincComponentCompiler.scala:137:26: Unused import
[warn] import sbt.io.Path.toURLs
[warn] ^
[warn] /Users/jz/code/zinc/internal/zinc-ivy-integration/src/main/scala/sbt/internal/inc/ZincComponentCompiler.scala:94:30: The outer reference in this type test cannot be checked at run time.
[warn] private final case class ScalaArtifacts(compiler: File, library: File, others: Vector[File])
[warn] ^
^C
[warn] Canceling execution...
[warn] four warnings found
[info] Compilation has been cancelled
[error] Total time: 1 s, completed 03/04/2018 2:24:35 PM
```
Best served with https://github.com/scala/scala/pull/6479Fixes#3958
Forward-port of #2303.
The checking code has bad run time characteristics and would need to be fixed
for large projects with deep task dependency chains.
The code in sbt.Execute has been in production for a long time so it seems safe
enough to drop the extra checks by default. To debug issues, you can set
`-Dsbt.execute.extrachecks=true` to revert to the old behavior.
Set sbt.task.timings=true to print timings for tasks.
This sample progress handler shows how to get names for tasks and
deal with flatMapped tasks. There are still some tasks that make
it through as anonymous, which needs to be investigated.
A setting to provide a custom handler should come in a subsequent commit.
1. KList[M[_]] now instead of KList[HL <: HList, M[_]]
a. head, tail work properly in this variant
b. disadvantage is that full type not easily transformed to new type constructor
2. AList abstracts on K[L[x]], a higher order type constructor.
A. Instances written for:
a. KList
b. Seq[M[T]] for a fixed T
c. TupleN
d. single values
e. operate on one type constructor when nested
B. Main disadvantage is type inference. It just doesn't happen for K[L[x]].
This is mitigated by AList being used internally and rarely needing to construct a K.
Eugene Yokota
Dale Wijnand
Jason Zaugg
xuwei-k
Johannes Rudolph
Martin Duhem
Pierre DAL-PRA
Jean-Rémi Desjardins
Josh Suereth
Mark Harrah
Grzegorz Kossakowski
Eugene Vigdorchik