Lazy in constructor parameter causes Nil$.head

[sir-wabbit]

object App {
  def main(args: Array[String]): Unit = {
    class T(lazy val a: Int)
    new T(1)
  }
}

////////////////////////////////////////////////////////

Exception in thread "main" java.util.NoSuchElementException: head of empty list
	at scala.collection.immutable.Nil$.head(List.scala:428)
	at scala.collection.immutable.Nil$.head(List.scala:425)
	at dotty.tools.dotc.transform.SymUtils$.loop$1(SymUtils.scala:83)
	at dotty.tools.dotc.transform.SymUtils$.subst$extension(SymUtils.scala:85)
	at dotty.tools.dotc.transform.Constructors.$anonfun$transformTemplate$3(Constructors.scala:230)
	at scala.collection.immutable.List.flatMap(List.scala:335)
	at dotty.tools.dotc.transform.Constructors.transformTemplate(Constructors.scala:222)
	at dotty.tools.dotc.transform.MegaPhase.goTemplate(MegaPhase.scala:906)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:318)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:365)
	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:234)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:364)
	at dotty.tools.dotc.transform.MegaPhase.transformStat$1(MegaPhase.scala:373)
	at dotty.tools.dotc.transform.MegaPhase.$anonfun$transformStats$2(MegaPhase.scala:378)
	at scala.collection.immutable.List.mapConserve(List.scala:176)
	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:378)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:264)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:365)
	at dotty.tools.dotc.transform.MegaPhase.mapDefDef$1(MegaPhase.scala:228)
	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:231)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:364)
	at dotty.tools.dotc.transform.MegaPhase.transformStat$1(MegaPhase.scala:373)
	at dotty.tools.dotc.transform.MegaPhase.$anonfun$transformStats$2(MegaPhase.scala:378)
	at scala.collection.immutable.List.mapConserve(List.scala:176)
	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:378)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:317)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:365)
	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:234)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:364)
	at dotty.tools.dotc.transform.MegaPhase.transformStat$1(MegaPhase.scala:373)
	at dotty.tools.dotc.transform.MegaPhase.$anonfun$transformStats$2(MegaPhase.scala:378)
	at scala.collection.immutable.List.mapConserve(List.scala:176)
	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:378)
	at dotty.tools.dotc.transform.MegaPhase.mapPackage$1(MegaPhase.scala:334)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:337)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:365)
	at dotty.tools.dotc.transform.MegaPhase.transformUnit(MegaPhase.scala:384)
	at dotty.tools.dotc.transform.MegaPhase.run(MegaPhase.scala:396)
	at dotty.tools.dotc.core.Phases$Phase.$anonfun$runOn$1(Phases.scala:293)
	at scala.collection.immutable.List.map(List.scala:283)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:291)
	at dotty.tools.dotc.core.Phases$Phase.runOn$(Phases.scala:290)
	at dotty.tools.dotc.transform.MegaPhase.runOn(MegaPhase.scala:138)
	at dotty.tools.dotc.Run.$anonfun$compileUnits$3(Run.scala:171)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.util.Stats$.trackTime(Stats.scala:47)
	at dotty.tools.dotc.Run.$anonfun$compileUnits$2(Run.scala:168)
	at dotty.tools.dotc.Run.$anonfun$compileUnits$2$adapted(Run.scala:166)
	at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:32)
	at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:29)
	at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:191)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:166)
	at dotty.tools.dotc.Run.$anonfun$compileUnits$1(Run.scala:191)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:88)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:149)
	at dotty.tools.dotc.Run.compileSources(Run.scala:136)
	at dotty.tools.dotc.Run.compile(Run.scala:120)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:29)
	at dotty.tools.dotc.Driver.process(Driver.scala:127)
	at dotty.tools.dotc.Driver.process(Driver.scala:96)
	at dotty.tools.dotc.Driver.process(Driver.scala:108)
	at dotty.tools.dotc.Driver.main(Driver.scala:135)
	at dotty.tools.dotc.Main.main(Main.scala)

[Blaisorblade]

TL;DR, Scalac complains that lazy modifier not allowed here. Use call-by-name parameters instead, while Dotty appears to lack this check (or not have it early enough?).

Taking inspiration from Compiler.scala, maybe this could be checked by a new miniphase before RefChecks (say, like CheckStatic?)

Looking at the trace from the final call, the first interesting (non-obviously-correct) line is Constructors.scala:230, because subst is correct if its arguments are lists of the same length, but the call in Constructors.scala:230 is

val param = acc.subst(accessors, paramSyms)

A breakpoint there shows that paramSyms contains val a while accessors contains method a and a@lzy1, causing the crash. Given how late is the Constructors miniphase, I'm confident the issue should be fixed earlier.

[Blaisorblade]

Let's see if I make any progress here.

[allanrenucci]

I don't think we should introduce a new mini-phase. The parser should reject it (like in scalac). Probably here: https://github.com/lampepfl/dotty/blob/765550305f945a0d53a93ff1fabc9de56a3f0386/compiler/src/dotty/tools/dotc/parsing/Parsers.scala#L1842

Simpler test case: class T(lazy val a: Int)

[Blaisorblade]

Can try that, might be even easier!