"... already has a symbol" assertion in the IDE when trying to complete "scala."

[smarter]

To reproduce:

1. sbt launchIDE
2. Type scala.
3. Boom:

java.lang.AssertionError: assertion failed: scala.languageFeature$experimental$$.macros$ already has a symbol
        at scala.Predef$.assert(Predef.scala:170)
        at dotty.tools.dotc.core.SymbolLoaders.enterNew(SymbolLoaders.scala:35)
        at dotty.tools.dotc.core.SymbolLoaders.enterModule(SymbolLoaders.scala:59)
        at dotty.tools.dotc.core.SymbolLoaders.enterClassAndModule(SymbolLoaders.scala:102)
        at dotty.tools.dotc.core.classfile.ClassfileParser.enterClassAndModule$1(ClassfileParser.scala:681)
        at dotty.tools.dotc.core.classfile.ClassfileParser.enterOwnInnerClasses$$anonfun$1(ClassfileParser.scala:690)
        at scala.compat.java8.JProcedure1.apply(JProcedure1.java:18)
        at scala.compat.java8.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.mutable.HashMap$$anon$2$$anonfun$foreach$3.apply(HashMap.scala:139)
        at scala.collection.mutable.HashMap$$anon$2$$anonfun$foreach$3.apply(HashMap.scala:139)
        at scala.collection.mutable.HashTable$class.foreachEntry(HashTable.scala:236)
        at scala.collection.mutable.HashMap.foreachEntry(HashMap.scala:40)
        at scala.collection.mutable.HashMap$$anon$2.foreach(HashMap.scala:139)
        at dotty.tools.dotc.core.classfile.ClassfileParser.enterOwnInnerClasses(ClassfileParser.scala:692)
        at dotty.tools.dotc.core.classfile.ClassfileParser.parseClass(ClassfileParser.scala:130)
        at dotty.tools.dotc.core.classfile.ClassfileParser.run(ClassfileParser.scala:59)
        at dotty.tools.dotc.core.ClassfileLoader.load(SymbolLoaders.scala:326)
        at dotty.tools.dotc.core.ClassfileLoader.doComplete(SymbolLoaders.scala:322)
        at dotty.tools.dotc.core.SymbolLoader.complete(SymbolLoaders.scala:272)
        at dotty.tools.dotc.core.SymDenotations$$anon$54.complete(SymDenotations.scala:1843)
        at dotty.tools.dotc.core.SymDenotations$SymDenotation.completeFrom(SymDenotations.scala:228)
        at dotty.tools.dotc.core.SymDenotations$SymDenotation.info(SymDenotations.scala:206)
        at dotty.tools.dotc.core.SymDenotations$SymDenotation.ensureCompleted(SymDenotations.scala:345)
        at dotty.tools.dotc.core.SymDenotations$SymDenotation.flags(SymDenotations.scala:149)
        at dotty.tools.dotc.core.SymDenotations$ModuleCompleter.complete(SymDenotations.scala:1881)
        at dotty.tools.dotc.core.SymDenotations$SymDenotation.completeFrom(SymDenotations.scala:228)
        at dotty.tools.dotc.core.SymDenotations$SymDenotation.info(SymDenotations.scala:206)
        at dotty.tools.dotc.core.SymDenotations$SymDenotation.isAccessibleFrom(SymDenotations.scala:707)
        at dotty.tools.dotc.interactive.Interactive$.completions$$anonfun$2$$anonfun$1(Interactive.scala:91)
        at dotty.tools.dotc.core.Denotations$SingleDenotation.altsWith(Denotations.scala:625)
        at dotty.tools.dotc.interactive.Interactive$.completions$$anonfun$2(Interactive.scala:91)
        at scala.compat.java8.JProcedure2.apply(JProcedure2.java:18)
        at scala.compat.java8.JProcedure2.apply(JProcedure2.java:10)
        at dotty.tools.dotc.core.Types$Type.memberDenots$$anonfun$1(Types.scala:673)
        at scala.compat.java8.JProcedure1.apply(JProcedure1.java:18)
        at scala.compat.java8.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.immutable.HashSet$HashSet1.foreach(HashSet.scala:316)
        at scala.collection.immutable.HashSet$HashTrieSet.foreach(HashSet.scala:972)
        at scala.collection.immutable.HashSet$HashTrieSet.foreach(HashSet.scala:972)
        at scala.collection.immutable.HashSet$HashTrieSet.foreach(HashSet.scala:972)
        at dotty.tools.dotc.core.Types$Type.memberDenots(Types.scala:673)
        at dotty.tools.dotc.interactive.Interactive$.completions(Interactive.scala:92)
        at dotty.tools.dotc.interactive.Interactive$.completions$$anonfun$1(Interactive.scala:73)
        at scala.collection.immutable.List.flatMap(List.scala:338)
        at dotty.tools.dotc.interactive.Interactive$.completions(Interactive.scala:84)
        at dotty.tools.languageserver.DottyLanguageServer.completion$$anonfun$1(DottyLanguageServer.scala:203)
        at dotty.tools.languageserver.DottyLanguageServer.liftedTree1$1(DottyLanguageServer.scala:108)
        at dotty.tools.languageserver.DottyLanguageServer.computeAsync$$anonfun$1(DottyLanguageServer.scala:113)
        at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:602)
        at java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:577)
        at java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:443)
        at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
        at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
        at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
        at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)

[smarter]

I'm on it, this seems to be related to scala/scala#5952