Scaladoc generation fails with -release 8 on JDK 11+

[SethTisue]

the problem is reproducible in both Scala 2.12.8 and 2.13.0, and both JDK 11 and 12

% java -version
openjdk version "11.0.4" 2019-07-16
OpenJDK Runtime Environment AdoptOpenJDK (build 11.0.4+11)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 11.0.4+11, mixed mode)
% echo "class C" > C.scala                                     
% /usr/local/scala/scala-2.13.0/bin/scaladoc -release 8 C.scala
Exception in thread "main" java.lang.AssertionError: assertion failed: 
  type AnyRef in java.lang
     while compiling: C.scala
        during phase: globalPhase=terminal, enteringPhase=typer
     library version: version 2.13.0
    compiler version: version 2.13.0
  reconstructed args: -release 8

  last tree to typer: Literal(Constant(()))
       tree position: line 1 of C.scala
            tree tpe: Unit
              symbol: null
           call site: constructor C in class C in package <empty>

== Source file context for tree position ==

     1 class C
     2 
	at scala.reflect.internal.SymbolTable.throwAssertionError(SymbolTable.scala:170)
	at scala.tools.nsc.doc.model.ModelFactory$modelCreation$.createTemplate(ModelFactory.scala:699)
	at scala.tools.nsc.doc.model.ModelFactory.makeMember0$1(ModelFactory.scala:794)
	at scala.tools.nsc.doc.model.ModelFactory.makeMember(ModelFactory.scala:807)
	at scala.tools.nsc.doc.model.ModelFactory$DocTemplateImpl.$anonfun$ownMembers$1(ModelFactory.scala:365)
	at scala.tools.nsc.doc.model.ModelFactory$DocTemplateImpl.<init>(ModelFactory.scala:365)
	at scala.tools.nsc.doc.model.ModelFactory$PackageImpl.<init>(ModelFactory.scala:474)
	at scala.tools.nsc.doc.model.ModelFactory$modelCreation$$anon$9.<init>(ModelFactory.scala:686)
	at scala.tools.nsc.doc.model.ModelFactory$modelCreation$.createTemplate(ModelFactory.scala:686)
	at scala.tools.nsc.doc.model.ModelFactory.makeMember0$1(ModelFactory.scala:794)
	at scala.tools.nsc.doc.model.ModelFactory.makeMember(ModelFactory.scala:807)
	at scala.tools.nsc.doc.model.ModelFactory$DocTemplateImpl.$anonfun$ownMembers$1(ModelFactory.scala:365)
	at scala.tools.nsc.doc.model.ModelFactory$DocTemplateImpl.<init>(ModelFactory.scala:365)
	at scala.tools.nsc.doc.model.ModelFactory$PackageImpl.<init>(ModelFactory.scala:474)
	at scala.tools.nsc.doc.model.ModelFactory$modelCreation$$anon$9.<init>(ModelFactory.scala:686)
	at scala.tools.nsc.doc.model.ModelFactory$modelCreation$.createTemplate(ModelFactory.scala:686)
	at scala.tools.nsc.doc.model.ModelFactory.makeMember0$1(ModelFactory.scala:794)
	at scala.tools.nsc.doc.model.ModelFactory.makeMember(ModelFactory.scala:807)
	at scala.tools.nsc.doc.model.ModelFactory$DocTemplateImpl.$anonfun$ownMembers$1(ModelFactory.scala:365)
	at scala.tools.nsc.doc.model.ModelFactory$DocTemplateImpl.<init>(ModelFactory.scala:365)
	at scala.tools.nsc.doc.model.ModelFactory$PackageImpl.<init>(ModelFactory.scala:474)
	at scala.tools.nsc.doc.model.ModelFactory$RootPackageImpl.<init>(ModelFactory.scala:482)
	at scala.tools.nsc.doc.model.ModelFactory$modelCreation$$anon$8.<init>(ModelFactory.scala:668)
	at scala.tools.nsc.doc.model.ModelFactory$modelCreation$.createTemplate(ModelFactory.scala:668)
	at scala.tools.nsc.doc.model.ModelFactory$modelCreation$.createRootPackage(ModelFactory.scala:614)
	at scala.tools.nsc.doc.model.ModelFactory$$anon$1.<init>(ModelFactory.scala:54)
	at scala.tools.nsc.doc.model.ModelFactory.makeModel(ModelFactory.scala:51)
	at scala.tools.nsc.doc.DocFactory.makeUniverse(DocFactory.scala:88)
	at scala.tools.nsc.doc.DocFactory.generate$1(DocFactory.scala:131)
	at scala.tools.nsc.doc.DocFactory.document(DocFactory.scala:138)
	at scala.tools.nsc.ScalaDoc.process(ScalaDoc.scala:47)
	at scala.tools.nsc.ScalaDoc$.main(ScalaDoc.scala:102)
	at scala.tools.nsc.ScalaDoc.main(ScalaDoc.scala)

this has come up in the community build a few times. it was also noticed by @retronym back in March 2018, akka/akka#24711 (comment)

the (obvious) workaround is to only enable -release 8 during regular compilation and not during Scaladoc generation

[SethTisue]

got re-reported and fixed at #11927 (fix: scala/scala#8849)