Compiler crash with scala.MatchError: trait mixed into object "_setter_" (dotty.tools.dotc.core.Names$DerivedName) - vals from tuple desugaring

[Sciss]

Minimized code

trait Companion {
  trait AbstractFoo {
    val (a, b) = {
      val av  = 1
      val bv  = 2
      (av, bv)
    }
  }
}

object Bar extends Companion {
  class FooImpl extends AbstractFoo
}

Output

[error] scala.MatchError: Companion$AbstractFoo$_setter_$Companion$AbstractFoo$$_=$1 (of class dotty.tools.dotc.core.Names$DerivedName)
[error] 	at dotty.tools.dotc.core.NameOps$.extension_fieldName(NameOps.scala:335)
[error] 	at dotty.tools.dotc.transform.SymUtils$.extension_field(SymUtils.scala:163)
[error] 	at dotty.tools.dotc.transform.Memoize.transformDefDef(Memoize.scala:145)
[error] 	at dotty.tools.dotc.transform.MegaPhase.goDefDef(MegaPhase.scala:974)
[error] 	at dotty.tools.dotc.transform.MegaPhase.goDefDef(MegaPhase.scala:975)
[error] 	at dotty.tools.dotc.transform.MegaPhase.goNamed$1(MegaPhase.scala:165)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformNode(MegaPhase.scala:211)
[error] 	at dotty.tools.dotc.transform.MegaPhase$MiniPhase.transformFollowing(MegaPhase.scala:127)
[error] 	at dotty.tools.dotc.transform.Mixin.setters$1$$anonfun$1(Mixin.scala:277)
[error] 	at scala.collection.immutable.List.map(List.scala:246)
[error] 	at dotty.tools.dotc.transform.Mixin.setters$2(Mixin.scala:277)
[error] 	at dotty.tools.dotc.transform.Mixin.$anonfun$5(Mixin.scala:301)
[error] 	at scala.collection.immutable.List.flatMap(List.scala:293)
[error] 	at dotty.tools.dotc.transform.Mixin.transformTemplate(Mixin.scala:302)
[error] 	at dotty.tools.dotc.transform.Mixin.transformTemplate(Mixin.scala:177)
[error] 	at dotty.tools.dotc.transform.MegaPhase.goTemplate(MegaPhase.scala:1004)
[error] 	at dotty.tools.dotc.transform.MegaPhase.goTemplate(MegaPhase.scala:1005)
[error] 	at dotty.tools.dotc.transform.MegaPhase.goTemplate(MegaPhase.scala:1005)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:363)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:429)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:256)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:427)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:437)
[error] 	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error] 	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:1061)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:442)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:362)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:429)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:256)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:427)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:437)
[error] 	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error] 	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:1061)
[error] 	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:1061)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:442)
[error] 	at dotty.tools.dotc.transform.MegaPhase.mapPackage$1(MegaPhase.scala:382)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:385)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:429)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformUnit(MegaPhase.scala:448)
[error] 	at dotty.tools.dotc.transform.MegaPhase.run(MegaPhase.scala:460)
[error] 	at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:296)
[error] 	at scala.collection.immutable.List.map(List.scala:246)
[error] 	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:297)
[error] 	at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:185)
[error] 	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] 	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] 	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
[error] 	at dotty.tools.dotc.Run.runPhases$5(Run.scala:195)
[error] 	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:203)
[error] 	at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
[error] 	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:67)
[error] 	at dotty.tools.dotc.Run.compileUnits(Run.scala:210)
[error] 	at dotty.tools.dotc.Run.compileSources(Run.scala:147)
[error] 	at dotty.tools.dotc.Run.compile(Run.scala:129)
[error] 	at dotty.tools.dotc.Driver.doCompile(Driver.scala:38)
[error] 	at dotty.tools.dotc.Driver.process(Driver.scala:194)
[error] 	at dotty.tools.dotc.Main.process(Main.scala)
[error] 	at xsbt.CachedCompilerImpl.run(CachedCompilerImpl.java:69)

[griggt]

Minimized further:

trait Foo:
  val (a, b) = (1, 2)

class FooImpl extends Foo

scala.MatchError: Foo$_setter_$Foo$$_=$1 (of class dotty.tools.dotc.core.Names$DerivedName) while compiling /src/dotty-issues/versions/src/main/scala/Main.scala
[error] ## Exception when compiling 1 sources to /src/dotty-issues/versions/target/scala-3.0.0-M1/classes
[error] scala.MatchError: Foo$_setter_$Foo$$_=$1 (of class dotty.tools.dotc.core.Names$DerivedName)
[error] dotty.tools.dotc.core.NameOps$.extension_fieldName(NameOps.scala:335)
[error] dotty.tools.dotc.transform.SymUtils$.extension_field(SymUtils.scala:163)
[error] dotty.tools.dotc.transform.Memoize.transformDefDef(Memoize.scala:145)

Looks to have been broken since 0.26.0-RC1 (#8652).

[smarter]

A val in a trait is iimplemented as a setter that is called in the class that implements the trait. Here there's actually three vals involved because of the val pattern which desugars to:

val $1$ =  (1, 2)
val a = $1$._1
val b = $1$._2

The problem is linked to this compiler-generated $1$ (implemented using UniqueName to get a fresh name each time), when generating a setter name corresponding to it we end up with:

Foo$_setter_$Foo$$_=$1

Semantically this is:

Foo[Qualified $_setter_$ Foo$$_=][Unique $ 1]

The weird part is that the name doesn't end with _= . What happens in detail is that in traitSetterName we go from:

Foo[Qualified $$ ][Unique $ 1]

to:

Foo[Qualified $_setter_$ Foo$$][Unique $ 1]

when calling expandedName(decl.owner, TraitSetterName), then we go to:

Foo[Qualified $_setter_$ Foo$$_=][Unique $ 1]

when calling setterName, even if we fixed fieldName to handle this sort of things, we would still be in trouble for Scala 2 compatibility (since Scala 2 classes can extend Scala 3 traits), so we need to ensure that the setter part of the name always gets added at the end, even if we have a semantic name with multiple parts. Do you think you could take a look at that @odersky ?

[smarter]

For comparison scalac generates:

val x$1 = (1, 2)
val a = $1$._1
val b = $1$._2

and ends up with these three setters in the trait:

def x$1_=(x: (Int, Int)): Unit
def Foo$_setter_$a_=(x: Int): Unit
def Foo$_setter_$b_=(x$1: Int): Unit

For some reason the setter for the freshly generated x$1 doesn't get an expanded name, is that something we need to imitate for binary compatibility ? /cc @sjrd

[smarter]

For some reason the setter for the freshly generated x$1 doesn't get an expanded name

... looks like this is just a pretty-printing bug in scalac, if I look in the bytecode I do see:

void Foo$_setter_$Foo$$x$1_$eq(Tuple2 var1)