Crash in backend in secondary constructor edge case

[szymon-rd]

Compiler version

3.4.0-RC1-bin-20231113-0dfe593-NIGHTLY

Minimized code

Minimized code from tests/init/neg/secondary-ctor4 test:

class A

class B {
  val a = new A

  class C(i: Int) {
    def this() = {
      this(1)
      class Inner() {
        println(a)
      }
    }
  }
}

Output

Error: Error while emitting /Users/srodziewicz/Projects/dotty/.scala-build/dotty_3200b05eac-ff28374a87/src_generated/main/xd.scala
assertion failed: Cannot emit primitive conversion from Lxd$_$B$C; to Lxd$_$B;

java.lang.AssertionError: assertion failed: Cannot emit primitive conversion from LB$C; to LB;
        at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
        at dotty.tools.backend.jvm.BCodeIdiomatic$JCodeMethodN.emitT2T(BCodeIdiomatic.scala:268)
        at dotty.tools.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.adapt(BCodeBodyBuilder.scala:1117)
        at dotty.tools.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genAdaptAndSendToDest(BCodeBodyBuilder.scala:497)
        at dotty.tools.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genLoadTo(BCodeBodyBuilder.scala:492)
        at dotty.tools.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genLoad(BCodeBodyBuilder.scala:305)
        at dotty.tools.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genStat(BCodeBodyBuilder.scala:111)
        at dotty.tools.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genBlockTo$$anonfun$1(BCodeBodyBuilder.scala:1095)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.immutable.List.foreach(List.scala:333)
        at dotty.tools.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genBlockTo(BCodeBodyBuilder.scala:1095)
        at dotty.tools.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genLoadTo(BCodeBodyBuilder.scala:459)
        at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.emitNormalMethodBody$1(BCodeSkelBuilder.scala:893)
        at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.genDefDef(BCodeSkelBuilder.scala:916)
  ...

[szymon-rd]

Crashes in 3.2 and 3.1 as well, not a regression

[szymon-rd]

Tree checker also doesnt like this snippet:

Exception in thread "main" dotty.tools.dotc.reporting.UnhandledError: Reassignment to val $outer
        at dotty.tools.dotc.reporting.ThrowingReporter.doReport(ThrowingReporter.scala:14)
        at dotty.tools.dotc.reporting.Reporter.issueUnconfigured(Reporter.scala:159)
        at dotty.tools.dotc.reporting.Reporter.go$1(Reporter.scala:185)
        at dotty.tools.dotc.reporting.Reporter.issueIfNotSuppressed(Reporter.scala:204)
        at dotty.tools.dotc.reporting.Reporter.report(Reporter.scala:207)
        at dotty.tools.dotc.report$.error(report.scala:68)
        at dotty.tools.dotc.typer.Typer.reassignmentToVal$1(Typer.scala:1097)
        at dotty.tools.dotc.typer.Typer.typedAssign(Typer.scala:1171)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3139)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3216)
        at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:174)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:398)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3293)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3297)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:381)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3346)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3365)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedStats(TreeChecker.scala:654)
        at dotty.tools.dotc.typer.Typer.typedBlockStats(Typer.scala:1182)
        at dotty.tools.dotc.typer.Typer.typedBlock(Typer.scala:1186)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedBlock$$anonfun$1$$anonfun$1(TreeChecker.scala:636)

[szymon-rd]

The cause

Here is a simplified version of the tree after constructors phase:

...
class C extends Object {
  ...
  def <init>($outer: B): Unit =
    {
      class Inner extends Object {
        def <init>($outer: B.this.C): Unit =
          {
            ...
            $outer = $outer
            ...
          }
        private val $outer: B.this.C
      }
      ()
    }
  private val $outer: B
}
...

The $outer = $outer comes from the fact that $outer is passed as the class arg to Inner. Constructors phase seems to be doing one thing wrong - it resolves the LHS to the $outer in C class, instead of Inner.

[szymon-rd]

Found the bug:

mapOuter has this transformer defined:

   override def transform(tree: Tree)(using Context) = tree match {
        ....
        case tree: RefTree if tree.symbol.is(ParamAccessor) && tree.symbol.name == nme.OUTER =>
          ref(outerParam)
       ...

The problem is that if there are any nested constructors in the constructor, they will also be transformed. And all will point to the same $outer.