By-Name trait parameters crash

[bohonghuang]

Minimized code

  trait A(l: => Any) {
    def f = l
  } // Error!
  //class B(l: => Any) {
  //  def f = l
  //} // OK

Output (click arrow to expand)

dotc: Error: head of empty list java.util.NoSuchElementException: head of empty list at scala.collection.immutable.Nil$.head(List.scala:629) at scala.collection.immutable.Nil$.head(List.scala:628) at dotty.tools.dotc.transform.SymUtils$.loop$1(SymUtils.scala:130) at dotty.tools.dotc.transform.SymUtils$.extension_subst(SymUtils.scala:132) at dotty.tools.dotc.transform.Constructors.$anonfun$3(Constructors.scala:258) at scala.collection.immutable.List.flatMap(List.scala:293) at dotty.tools.dotc.transform.Constructors.transformTemplate(Constructors.scala:275) at dotty.tools.dotc.transform.MegaPhase.goTemplate(MegaPhase.scala:1003) at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:362) at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:428) at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:255) at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:426) at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:436) at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:441) at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:1060) at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:1060) at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:441) at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:361) at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:428) at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:255) at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:426) at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:436) at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:441) at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:1060) at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:441) at dotty.tools.dotc.transform.MegaPhase.mapPackage$1(MegaPhase.scala:381) at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:384) at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:428) at dotty.tools.dotc.transform.MegaPhase.transformUnit(MegaPhase.scala:447) at dotty.tools.dotc.transform.MegaPhase.run(MegaPhase.scala:459) at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:296) at scala.collection.immutable.List.map(List.scala:246) at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:297) at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:180) at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15) at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10) at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323) at dotty.tools.dotc.Run.runPhases$5(Run.scala:190) at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:198) at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12) at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:64) at dotty.tools.dotc.Run.compileUnits(Run.scala:205) at dotty.tools.dotc.Run.compileSources(Run.scala:142) at dotty.tools.dotc.Run.compile(Run.scala:124) at dotty.tools.dotc.Driver.doCompile(Driver.scala:38) at dotty.tools.dotc.Driver.process(Driver.scala:195) at dotty.tools.dotc.Main.process(Main.scala) at xsbt.CachedCompilerImpl.run(CachedCompilerImpl.java:69) at xsbt.CompilerInterface.run(CompilerInterface.java:41) 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 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)