Compiler crash when compiling value classes with an underlying private field

[hamzaremmal]

Compiler version

All Scala 3 versions

Minimized code

trait Bar[T]
class Foo[T <: Bar[T]] (private val buffer: Any) extends AnyVal

Output

java.lang.AssertionError: assertion failed: no erasure for <notype>
	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
	at dotty.tools.dotc.core.TypeErasure.eraseDerivedValueClass(TypeErasure.scala:859)
	at dotty.tools.dotc.core.TypeErasure.dotty$tools$dotc$core$TypeErasure$$apply(TypeErasure.scala:645)
	at dotty.tools.dotc.core.TypeErasure$.valueErasure(TypeErasure.scala:223)
	at dotty.tools.dotc.core.TypeErasure$.fullErasure(TypeErasure.scala:231)
	at dotty.tools.dotc.transform.GenericSignatures$.jsig$1(GenericSignatures.scala:278)
	at dotty.tools.dotc.transform.GenericSignatures$.jsig$1$$anonfun$1(GenericSignatures.scala:302)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:334)
	at dotty.tools.dotc.transform.GenericSignatures$.jsig$1(GenericSignatures.scala:302)
	at dotty.tools.dotc.transform.GenericSignatures$.javaSig0(GenericSignatures.scala:341)
	at dotty.tools.dotc.transform.GenericSignatures$.javaSig(GenericSignatures.scala:40)
	at dotty.tools.backend.jvm.BCodeHelpers.dotty$tools$backend$jvm$BCodeHelpers$$getGenericSignatureHelper(BCodeHelpers.scala:777)
	at dotty.tools.backend.jvm.BCodeHelpers.dotty$tools$backend$jvm$BCodeHelpers$$getStaticForwarderGenericSignature(BCodeHelpers.scala:842)
	at dotty.tools.backend.jvm.BCodeHelpers$BCForwardersGen.addForwarder(BCodeHelpers.scala:434)
	at dotty.tools.backend.jvm.BCodeHelpers$BCForwardersGen.addForwarders$$anonfun$3(BCodeHelpers.scala:514)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:334)
	at dotty.tools.backend.jvm.BCodeHelpers$BCForwardersGen.addForwarders(BCodeHelpers.scala:493)
	at dotty.tools.backend.jvm.BCodeHelpers$BCForwardersGen.addForwarders$(BCodeHelpers.scala:410)
	at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.addForwarders(BCodeSkelBuilder.scala:134)
	at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.initJClass(BCodeSkelBuilder.scala:362)
	at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.genPlainClass(BCodeSkelBuilder.scala:183)
	at dotty.tools.backend.jvm.CodeGen.genClass(CodeGen.scala:161)
	at dotty.tools.backend.jvm.CodeGen.genClassDef$1(CodeGen.scala:62)
	at dotty.tools.backend.jvm.CodeGen.genClassDefs$1(CodeGen.scala:118)
	at dotty.tools.backend.jvm.CodeGen.genClassDefs$1$$anonfun$1(CodeGen.scala:116)
	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:334)
	at dotty.tools.backend.jvm.CodeGen.genClassDefs$1(CodeGen.scala:116)
	at dotty.tools.backend.jvm.CodeGen.genUnit(CodeGen.scala:121)
	at dotty.tools.backend.jvm.GenBCode.run(GenBCode.scala:90)
	at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:383)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:334)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:376)
	at dotty.tools.backend.jvm.GenBCode.runOn(GenBCode.scala:98)
	at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:345)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1324)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:338)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:385)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:397)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:69)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:397)
	at dotty.tools.dotc.Run.compileSources(Run.scala:284)
	at dotty.tools.dotc.Run.compile(Run.scala:269)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:37)
	at dotty.tools.dotc.Driver.process(Driver.scala:201)
	at dotty.tools.dotc.Driver.process(Driver.scala:169)
	at dotty.tools.dotc.Driver.process(Driver.scala:181)
	at dotty.tools.dotc.Driver.main(Driver.scala:211)
	at dotty.tools.dotc.Main.main(Main.scala)
Error while emitting s.scala
assertion failed: no erasure for <notype>

Expectation

To compile

Notes

If we print the trees using -Xprint:all, the compilation is successful. The issue was minimized from a community build failure in #22519. I believe that the full parametrization fails to generate a correct type when synthesizing the equals$extension method.
Also, SIP-15 prohibited private underlying fields in value classes, but this was relaxed overtime.