Assertion failure in TypeErasure#sigName: "no sig for MethodType"

[griggt]

Minimized from the community-build failure of zio/zio-config reported in #15647

Compiler version

3.2.0-RC2
Works on 3.1.3
Fails on 3.2.0-RC1
Fails on 3.2.1-RC1-bin-20220710-794e7c9-NIGHTLY

Minimized code

trait ConfigSourceModule:
  object ConfigSource:
    class R

object M extends ConfigSourceModule

object Foo:
  implicit class FromConfigSource(c: M.ConfigSource.type)

object FooBar:                                      // problem disappears if we rename as `Bar`
  def foo: M.ConfigSource.R = new M.ConfigSource.R  // problem disappears if we use `???` as rhs

Output (click arrow to expand)

java.lang.AssertionError: assertion failed: MethodType(List(), List(), TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,module class <empty>)),trait ConfigSourceModule)),module class ConfigSource$))
	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
	at dotty.tools.dotc.core.TypeErasure.dotty$tools$dotc$core$TypeErasure$$sigName(TypeErasure.scala:851)
	at dotty.tools.dotc.core.TypeErasure$.sigName(TypeErasure.scala:204)
	at dotty.tools.dotc.core.Signature.$anonfun$2(Signature.scala:111)
	at scala.collection.immutable.List.map(List.scala:246)
	at dotty.tools.dotc.core.Signature.prependTermParams(Signature.scala:111)
	at dotty.tools.dotc.core.Types$MethodOrPoly.computeSignature$2(Types.scala:3560)
	at dotty.tools.dotc.core.Types$MethodOrPoly.signature(Types.scala:3577)
	at dotty.tools.dotc.core.Types$MethodOrPoly.signature(Types.scala:3587)
	at dotty.tools.dotc.core.TypeComparer.compareMethod$1(TypeComparer.scala:703)
	at dotty.tools.dotc.core.TypeComparer.thirdTry$1(TypeComparer.scala:709)
	at dotty.tools.dotc.core.TypeComparer.secondTry$1(TypeComparer.scala:506)
	at dotty.tools.dotc.core.TypeComparer.firstTry$1(TypeComparer.scala:383)
	at dotty.tools.dotc.core.TypeComparer.recur(TypeComparer.scala:1336)
	at dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:194)
	at dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:204)
	at dotty.tools.dotc.core.TypeComparer.isSameType(TypeComparer.scala:2019)
	at dotty.tools.dotc.core.TypeComparer$.isSameType(TypeComparer.scala:2761)
	at dotty.tools.dotc.core.Types$Type.$eq$colon$eq(Types.scala:1056)
	at dotty.tools.backend.jvm.BCodeHelpers.dotty$tools$backend$jvm$BCodeHelpers$$getStaticForwarderGenericSignature(BCodeHelpers.scala:920)
	at dotty.tools.backend.jvm.BCodeHelpers$BCForwardersGen.addForwarder(BCodeHelpers.scala:514)
	at dotty.tools.backend.jvm.BCodeHelpers$BCForwardersGen.addForwarders$$anonfun$3(BCodeHelpers.scala:594)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.backend.jvm.BCodeHelpers$BCForwardersGen.addForwarders(BCodeHelpers.scala:596)
	at dotty.tools.backend.jvm.BCodeHelpers$BCForwardersGen.addForwarders$(BCodeHelpers.scala:490)
	at dotty.tools.backend.jvm.BCodeHelpers$JCommonBuilder.addForwarders(BCodeHelpers.scala:654)
	at dotty.tools.backend.jvm.BCodeHelpers$JMirrorBuilder.genMirrorClass(BCodeHelpers.scala:701)
	at dotty.tools.backend.jvm.GenBCodePipeline$Worker1.visit(GenBCode.scala:257)
	at dotty.tools.backend.jvm.GenBCodePipeline$Worker1.run(GenBCode.scala:231)
	at dotty.tools.backend.jvm.GenBCodePipeline.buildAndSendToDisk(GenBCode.scala:598)
	at dotty.tools.backend.jvm.GenBCodePipeline.run(GenBCode.scala:564)
	at dotty.tools.backend.jvm.GenBCode.run(GenBCode.scala:69)
	at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:311)
	at scala.collection.immutable.List.map(List.scala:246)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:312)
	at dotty.tools.backend.jvm.GenBCode.runOn(GenBCode.scala:77)
	at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:234)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1328)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:245)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:253)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:262)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:262)
	at dotty.tools.dotc.Run.compileSources(Run.scala:186)
	at dotty.tools.dotc.Run.compile(Run.scala:170)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:35)
	at dotty.tools.dotc.Driver.process(Driver.scala:195)
	at dotty.tools.dotc.Driver.process(Driver.scala:163)
	at dotty.tools.dotc.Driver.process(Driver.scala:175)
	at dotty.tools.dotc.Driver.main(Driver.scala:205)
	at dotty.tools.dotc.Main.main(Main.scala)
no sig for MethodType(List(), List(), TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,module class <empty>)),trait ConfigSourceModule)),module class ConfigSource$)) because of ()
assertion failure for (c(): ConfigSourceModule.this.ConfigSource): Foo.FromConfigSource <:< (c: ConfigSourceModule.this.ConfigSource): Foo.FromConfigSource, frozen = false
Error while emitting ConfigSourceModule.scala
exception occurred while compiling ConfigSourceModule.scala
java.lang.AssertionError: assertion failed: MethodType(List(), List(), TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,module class <empty>)),trait ConfigSourceModule)),module class ConfigSource$)) while compiling ConfigSourceModule.scala
Exception in thread "main" java.lang.AssertionError: assertion failed: MethodType(List(), List(), TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,module class <empty>)),trait ConfigSourceModule)),module class ConfigSource$))
	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
	at dotty.tools.dotc.core.TypeErasure.dotty$tools$dotc$core$TypeErasure$$sigName(TypeErasure.scala:851)
	at dotty.tools.dotc.core.TypeErasure$.sigName(TypeErasure.scala:204)
	at dotty.tools.dotc.core.Signature.$anonfun$2(Signature.scala:111)
	at scala.collection.immutable.List.map(List.scala:246)
	at dotty.tools.dotc.core.Signature.prependTermParams(Signature.scala:111)
	at dotty.tools.dotc.core.Types$MethodOrPoly.computeSignature$2(Types.scala:3560)
	at dotty.tools.dotc.core.Types$MethodOrPoly.signature(Types.scala:3577)
	at dotty.tools.dotc.core.Types$MethodOrPoly.signature(Types.scala:3587)
	at dotty.tools.dotc.core.TypeComparer.compareMethod$1(TypeComparer.scala:703)
	at dotty.tools.dotc.core.TypeComparer.thirdTry$1(TypeComparer.scala:709)
	at dotty.tools.dotc.core.TypeComparer.secondTry$1(TypeComparer.scala:506)
	at dotty.tools.dotc.core.TypeComparer.firstTry$1(TypeComparer.scala:383)
	at dotty.tools.dotc.core.TypeComparer.recur(TypeComparer.scala:1336)
	at dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:194)
	at dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:204)
	at dotty.tools.dotc.core.TypeComparer.isSameType(TypeComparer.scala:2019)
	at dotty.tools.dotc.core.TypeComparer$.isSameType(TypeComparer.scala:2761)
	at dotty.tools.dotc.core.Types$Type.$eq$colon$eq(Types.scala:1056)
	at dotty.tools.backend.jvm.BCodeHelpers.dotty$tools$backend$jvm$BCodeHelpers$$getStaticForwarderGenericSignature(BCodeHelpers.scala:920)
	at dotty.tools.backend.jvm.BCodeHelpers$BCForwardersGen.addForwarder(BCodeHelpers.scala:514)
	at dotty.tools.backend.jvm.BCodeHelpers$BCForwardersGen.addForwarders$$anonfun$3(BCodeHelpers.scala:594)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.backend.jvm.BCodeHelpers$BCForwardersGen.addForwarders(BCodeHelpers.scala:596)
	at dotty.tools.backend.jvm.BCodeHelpers$BCForwardersGen.addForwarders$(BCodeHelpers.scala:490)
	at dotty.tools.backend.jvm.BCodeHelpers$JCommonBuilder.addForwarders(BCodeHelpers.scala:654)
	at dotty.tools.backend.jvm.BCodeHelpers$JMirrorBuilder.genMirrorClass(BCodeHelpers.scala:701)
	at dotty.tools.backend.jvm.GenBCodePipeline$Worker1.visit(GenBCode.scala:257)
	at dotty.tools.backend.jvm.GenBCodePipeline$Worker1.run(GenBCode.scala:231)
	at dotty.tools.backend.jvm.GenBCodePipeline.buildAndSendToDisk(GenBCode.scala:598)
	at dotty.tools.backend.jvm.GenBCodePipeline.run(GenBCode.scala:564)
	at dotty.tools.backend.jvm.GenBCode.run(GenBCode.scala:69)
	at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:311)
	at scala.collection.immutable.List.map(List.scala:246)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:312)
	at dotty.tools.backend.jvm.GenBCode.runOn(GenBCode.scala:77)
	at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:234)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1328)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:245)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:253)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:262)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:262)
	at dotty.tools.dotc.Run.compileSources(Run.scala:186)
	at dotty.tools.dotc.Run.compile(Run.scala:170)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:35)
	at dotty.tools.dotc.Driver.process(Driver.scala:195)
	at dotty.tools.dotc.Driver.process(Driver.scala:163)
	at dotty.tools.dotc.Driver.process(Driver.scala:175)
	at dotty.tools.dotc.Driver.main(Driver.scala:205)
	at dotty.tools.dotc.Main.main(Main.scala)

[griggt]

It seems as though there may be some non-determinism involved in this failure. While attempting to bisect it, it intermittently stopped reproducing (I've seen this behavior on Scastie as well, here's a run where it failed: https://scastie.scala-lang.org/OvLSO3O5SHCnicilO8PHrg).

[odersky]

I compiled this on main once, compiled OK.
A second compile crashed with a stacktrace like the one shown.

I then cleaned the output directory (which is on my classpath) and compiled another 10 times. Everything compiled OK.

Which makes me think that this is somehow sensitive to some unrelated class file on the class path.

It would be good to find the commit that caused this, even though with the flakey behavior it will be diffcult.

[odersky]

Yes, it looks highly non-deterministic. I could find out that it sees a MethodType that has another MethodType as parameter type, which makes no sense at all. But when I try to add an assertion to find out where it happens I can't make it crash again...

[griggt]

I spent some more time attempting to track this down. Today I used a significantly faster machine (i5-12400 vs. i5-4590s) and older JDK (8 vs. 17). For whatever reason, the behavior seemed more stable and the effort was more productive.

I was able to replicate the crash as early as ae5f986 (the merge commit for #15129), but despite repeated efforts could not get it to crash on the parent merge commit a293d7d.

Diving into the individual commits of that PR, it crashed on e609851 but not a690de2.

So my best guess is the first bad commit is e609851 from #15129.

[smarter]

So my best guess is the first bad commit is e609851 from #15129.

I did worry about this sort of things at the time: #15129 (comment) :)