Assertions with -Ycheck:all

[check: kind-projector] 1 new symbols.
[check: kind-projector] package scala
Caught java.lang.AssertionError: assertion failed: <none>
semigroup.scala
java.lang.AssertionError: assertion failed: <none>
	at scala.reflect.internal.SymbolTable.throwAssertionError(SymbolTable.scala:163)
	at scala.tools.nsc.typechecker.Typers$Typer.typedPackageDef$1(Typers.scala:5260)
	at scala.tools.nsc.typechecker.Typers$Typer.typedMemberDef$1(Typers.scala:5555)
	at scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:5602)
	at scala.tools.nsc.typechecker.Typers$Typer.runTyper$1(Typers.scala:5640)
	at scala.tools.nsc.typechecker.Typers$Typer.typedInternal(Typers.scala:5672)
	at scala.tools.nsc.typechecker.Typers$Typer.body$2(Typers.scala:5613)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:5618)
	at scala.tools.nsc.typechecker.TreeCheckers$TreeChecker.typed(TreeCheckers.scala:256)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:5699)
	at scala.tools.nsc.typechecker.TreeCheckers.$anonfun$check$1(TreeCheckers.scala:213)
	at scala.tools.nsc.typechecker.TreeCheckers.runWithUnit(TreeCheckers.scala:200)
	at scala.tools.nsc.typechecker.TreeCheckers.check(TreeCheckers.scala:211)
	at scala.tools.nsc.typechecker.TreeCheckers.$anonfun$checkTrees$3(TreeCheckers.scala:194)
	at scala.tools.nsc.typechecker.TreeCheckers.$anonfun$checkTrees$1(TreeCheckers.scala:181)
	at scala.tools.nsc.typechecker.TreeCheckers.$anonfun$checkTrees$1$adapted(TreeCheckers.scala:194)
	at scala.collection.Iterator.foreach(Iterator.scala:944)
	at scala.collection.Iterator.foreach$(Iterator.scala:944)
	at scala.collection.AbstractIterator.foreach(Iterator.scala:1432)
	at scala.tools.nsc.typechecker.TreeCheckers.checkTrees(TreeCheckers.scala:194)
	at scala.tools.nsc.Global$Run.runCheckers(Global.scala:1366)
	at scala.tools.nsc.Global$Run.compileUnitsInternal(Global.scala:1477)
	at scala.tools.nsc.Global$Run.compileUnits(Global.scala:1430)
	at scala.tools.nsc.Global$Run.compileSources(Global.scala:1423)
	at scala.tools.nsc.Global$Run.compile(Global.scala:1539)
	at xsbt.CachedCompiler0.run(CompilerInterface.scala:130)
	at xsbt.CachedCompiler0.run(CompilerInterface.scala:105)
	at xsbt.CompilerInterface.run(CompilerInterface.scala:31)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at sbt.internal.inc.AnalyzingCompiler.call(AnalyzingCompiler.scala:237)
	at sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:111)
	at sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:90)
	at sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$3(MixedAnalyzingCompiler.scala:83)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at sbt.internal.inc.MixedAnalyzingCompiler.timed(MixedAnalyzingCompiler.scala:134)
	at sbt.internal.inc.MixedAnalyzingCompiler.compileScala$1(MixedAnalyzingCompiler.scala:74)
	at sbt.internal.inc.MixedAnalyzingCompiler.compile(MixedAnalyzingCompiler.scala:117)
	at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1(IncrementalCompilerImpl.scala:305)
	at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1$adapted(IncrementalCompilerImpl.scala:305)
	at sbt.internal.inc.Incremental$.doCompile(Incremental.scala:101)
	at sbt.internal.inc.Incremental$.$anonfun$compile$4(Incremental.scala:82)
	at sbt.internal.inc.IncrementalCommon.recompileClasses(IncrementalCommon.scala:110)
	at sbt.internal.inc.IncrementalCommon.cycle(IncrementalCommon.scala:57)
	at sbt.internal.inc.Incremental$.$anonfun$compile$3(Incremental.scala:84)
	at sbt.internal.inc.Incremental$.manageClassfiles(Incremental.scala:129)
	at sbt.internal.inc.Incremental$.compile(Incremental.scala:75)
	at sbt.internal.inc.IncrementalCompile$.apply(Compile.scala:61)
	at sbt.internal.inc.IncrementalCompilerImpl.compileInternal(IncrementalCompilerImpl.scala:309)
	at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileIncrementally$1(IncrementalCompilerImpl.scala:267)
	at sbt.internal.inc.IncrementalCompilerImpl.handleCompilationError(IncrementalCompilerImpl.scala:158)
	at sbt.internal.inc.IncrementalCompilerImpl.compileIncrementally(IncrementalCompilerImpl.scala:237)
	at sbt.internal.inc.IncrementalCompilerImpl.compile(IncrementalCompilerImpl.scala:68)
	at sbt.Defaults$.compileIncrementalTaskImpl(Defaults.scala:1443)
	at sbt.Defaults$.$anonfun$compileIncrementalTask$1(Defaults.scala:1417)
	at scala.Function1.$anonfun$compose$1(Function1.scala:44)
	at sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:39)
	at sbt.std.Transform$$anon$4.work(System.scala:66)
	at sbt.Execute.$anonfun$submit$2(Execute.scala:263)
	at sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:16)
	at sbt.Execute.work(Execute.scala:272)
	at sbt.Execute.$anonfun$submit$1(Execute.scala:263)
	at sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:174)
	at sbt.CompletionService$$anon$2.call(CompletionService.scala:37)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)

Start by reproducing the assertion with -Ycheck:all using the semigroup.scala example. Read the Scala compiler typer and TreeCheckers frames shown in the stack trace, especially typedPackageDef, to locate the failure. Done means the example no longer triggers the reported AssertionError.