NoSuchMethodError when pattern matching on inner class (no outer accessor)

[SethTisue]

tested on both 3.0.0 and 3.0.1-RC2

originally reported by Scala book author @cayhorstmann at scala/scala-xml#541

Minimized code

class C1 {
  private class C2
  new C2 match {
    case c: C2 =>
  }
}

object Test extends App {
  new C1
}

Output

java.lang.NoSuchMethodError: C1$C2.C1$C2$$$outer()LC1;
 	at C1.<init>(S.scala:6)

which occurs because C2 simply doesn't have an outer accessor method. C2s constructor accepts an outer reference, but doesn't do anything with it:

  public C1$C2(C1);
    descriptor: (LC1;)V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=2, args_size=2
         0: aload_0
         1: invokespecial #14                 // Method java/lang/Object."<init>":()V
         4: return

Expectation

The runtime error occurs regardless of whether C2 is declared private[this] or merely private. It stops occurring if you declare C2 to be private[C1]. (Perhaps the private version fails because of the Scala 3 behavior where plain private sometimes means private[this] and sometimes means private[C1]?)

Ironically, Scala 2 does the reverse: for private[this] it gives C2 an outer accessor, but then the pattern match in C1's constructor does not call it...!

So, what would the correct behavior — keep the check and make it work, or omit it? I don't know offhand, especially once you start considering the nuances of the different access levels the inner class might have. But I see there is a lot of previous lore in this area: #2156, scala/bug#4440, scala/bug#1419, maybe others...?

[dwijnand]

So, what would the correct behavior — keep the check and make it work, or omit it?

Make it work, IMO. Looks like it's in one of posttyper's phases (oh, probably explicitOuter) that isn't capturing an outer reference in private class C2.

[SethTisue]

compilation of the test code fails under -Ycheck:all with

checking tests/run/i13096.scala after phase MegaPhase{elimOpaque, tryCatchPatterns, patternMatcher, explicitOuter, explicitSelf, elimByName, stringInterpolatorOpt}
exception while typing private[this] class C2() extends Object() {} of class class dotty.tools.dotc.ast.Trees$TypeDef # -1
exception while typing @scala.annotation.internal.SourceFile("tests/run/i13096.scala") class C1()
...
*** error while checking tests/run/i13096.scala after phase stringInterpolatorOpt ***
exception occurred while compiling tests/run/i13096.scala
java.lang.AssertionError: assertion failed:  class C2 tree does not define members: method C1$C2$$$outer, value $outer
expected: constructor C2, method C1$C2$$$outer, value $outer
defined:  while compiling tests/run/i13096.scala
Exception in thread "main" java.lang.AssertionError: assertion failed:  class C2 tree does not define members: method C1$C2$$$outer, value $outer
expected: constructor C2, method C1$C2$$$outer, value $outer
defined: 
	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedClassDef(TreeChecker.scala:482)
	at dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$2(Typer.scala:2681)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2685)

[SethTisue]

PR: #13124