Assertion fail during implicit search: constraint before search neq constraint after search

[allanrenucci]

The following code snippet crashes the compiler:

class Test {
  import Test.test
  "Hello".toto
}

object Test {
  def test = {
    implicitly[collection.generic.CanBuildFrom[List[_], Int, List[_]]]
  }
}

Stacktrace

exception occurred while typechecking tests/allan/Test.scala
exception occurred while compiling tests/allan/Test.scala

Exception in thread "main" java.lang.AssertionError: assertion failed

at scala.Predef$.assert(Predef.scala:204)

at dotty.tools.dotc.typer.Implicits.$anonfun$inferImplicit$4(Implicits.scala:740)

at dotty.tools.dotc.reporting.trace$.apply(trace.scala:23)

at dotty.tools.dotc.typer.Implicits.$anonfun$inferImplicit$1(Implicits.scala:712)

at dotty.tools.dotc.util.Stats$.track(Stats.scala:35)

at dotty.tools.dotc.typer.Implicits.inferImplicit(Implicits.scala:707)

at dotty.tools.dotc.typer.Implicits.inferImplicit$(Implicits.scala:707)

at dotty.tools.dotc.typer.Typer.inferImplicit(Typer.scala:69)

at dotty.tools.dotc.typer.Implicits.$anonfun$inferView$1(Implicits.scala:537)

at dotty.tools.dotc.util.Stats$.track(Stats.scala:35)

at dotty.tools.dotc.typer.Implicits.inferView(Implicits.scala:524)

at dotty.tools.dotc.typer.Implicits.inferView$(Implicits.scala:523)

at dotty.tools.dotc.typer.Typer.inferView(Typer.scala:69)

at dotty.tools.dotc.typer.Typer.adaptToSubType$1(Typer.scala:2228)

at dotty.tools.dotc.typer.Typer.adaptNoArgsOther$1(Typer.scala:2136)

at dotty.tools.dotc.typer.Typer.adaptNoArgs$1(Typer.scala:2175)

at dotty.tools.dotc.typer.Typer.adaptInterpolated(Typer.scala:2298)

at dotty.tools.dotc.typer.Typer.$anonfun$adapt$3(Typer.scala:1862)

at dotty.tools.dotc.reporting.trace$.apply(trace.scala:23)

at dotty.tools.dotc.typer.Typer.$anonfun$adapt$1(Typer.scala:1858)

at dotty.tools.dotc.util.Stats$.track(Stats.scala:35)

at dotty.tools.dotc.typer.Typer.adapt(Typer.scala:1858)

at dotty.tools.dotc.typer.Typer.$anonfun$typed$2(Typer.scala:1696)

at dotty.tools.dotc.reporting.trace$.apply(trace.scala:23)

at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1692)

at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:1753)

at dotty.tools.dotc.typer.Typer.typeSelectOnTerm$1(Typer.scala:397)

at dotty.tools.dotc.typer.Typer.$anonfun$typedSelect$1(Typer.scala:431)

at dotty.tools.dotc.util.Stats$.track(Stats.scala:35)

at dotty.tools.dotc.typer.Typer.typedSelect(Typer.scala:395)

at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:1613)

at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1678)

at dotty.tools.dotc.typer.Typer.$anonfun$typed$2(Typer.scala:1696)

at dotty.tools.dotc.reporting.trace$.apply(trace.scala:23)

at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1692)

at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:1729)

at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:1741)

at dotty.tools.dotc.typer.Typer.$anonfun$typedClassDef$1(Typer.scala:1409)

at dotty.tools.dotc.util.Stats$.track(Stats.scala:35)

at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:1339)

at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:1623)

at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1678)

at dotty.tools.dotc.typer.Typer.$anonfun$typed$2(Typer.scala:1696)

at dotty.tools.dotc.reporting.trace$.apply(trace.scala:23)

at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1692)

at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:1718)

at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:1741)

at dotty.tools.dotc.typer.Typer.$anonfun$typedPackageDef$1(Typer.scala:1522)

at dotty.tools.dotc.util.Stats$.track(Stats.scala:35)

at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:1509)

at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1662)

at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1679)

at dotty.tools.dotc.typer.Typer.$anonfun$typed$2(Typer.scala:1696)

at dotty.tools.dotc.reporting.trace$.apply(trace.scala:23)

at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1692)

at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:1753)

at dotty.tools.dotc.typer.FrontEnd.$anonfun$typeCheck$1(FrontEnd.scala:64)

at dotty.tools.dotc.typer.FrontEnd.monitor(FrontEnd.scala:32)

at dotty.tools.dotc.typer.FrontEnd.typeCheck(FrontEnd.scala:62)

at dotty.tools.dotc.typer.FrontEnd.$anonfun$runOn$7(FrontEnd.scala:93)

at dotty.tools.dotc.typer.FrontEnd.$anonfun$runOn$7$adapted(FrontEnd.scala:93)

at scala.collection.immutable.List.foreach(List.scala:389)

at dotty.tools.dotc.typer.FrontEnd.runOn(FrontEnd.scala:93)

at dotty.tools.dotc.Run.$anonfun$compileUnits$3(Run.scala:124)

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:122)

at dotty.tools.dotc.Run.$anonfun$compileUnits$2$adapted(Run.scala:120)

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.$anonfun$compileUnits$1(Run.scala:120)

at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)

at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:86)

at dotty.tools.dotc.Run.compileUnits(Run.scala:108)

at dotty.tools.dotc.Run.compileSources(Run.scala:95)

at dotty.tools.dotc.Run.compile(Run.scala:79)

at dotty.tools.dotc.Driver.doCompile(Driver.scala:26)

at dotty.tools.dotc.Driver.process(Driver.scala:124)

at dotty.tools.dotc.Driver.process(Driver.scala:93)

at dotty.tools.dotc.Driver.process(Driver.scala:105)

at dotty.tools.dotc.Driver.main(Driver.scala:132)

at dotty.tools.dotc.Main.main(Main.scala)

[allanrenucci]

My understanding of the problem:

The compiler performs an implicit search to find a method toto on String. To do so, it typechecks Test.test which modifies ctx.typerState.constraint and reference equality is lost between ctx.typerState.constraint before and after the search.

[odersky]

Unfortunately, #3354 did have a performance impact. I think we should try to revert the change, not create a new typerState, and just switch back to the old context in case of failure.

[liufengyun]

@odersky From the following benchmarks, it seems #3354 has no impact on performance. Are you checking the benchmarks from your local machine?

http://dotty-bench.epfl.ch/3354/