Avoid stackoverflow in ExplicitOuter (#16381)

When transforming a class at ExplicitOuter we create outer accessors for
it. The newSymbol call to do this takes place at phase ExplicitOuter +
1, but its arguments need to be evaluated at phase ExplicitOuter. This
was not true for the nestingLevel argument, which demanded the
denotation of the class at phase ExplicitOuter + 1, thus leading to the
SO.

Interestingly, the same path is not taken if the class has all abstract
members defined or is declared abstract. It's only in the error case
that I could reproduce the SO.

Fixes #16343

Author: odersky

compiler/src/dotty/tools/dotc/transform/ExplicitOuter.scala

@@ -176,8 +176,9 @@ object ExplicitOuter {
           if prefix == NoPrefix then outerCls.typeRef.appliedTo(outerCls.typeParams.map(_ => TypeBounds.empty))
           else prefix.widen)
     val info = if (flags.is(Method)) ExprType(target) else target
+    val currentNestingLevel = ctx.nestingLevel
     atPhaseNoEarlier(explicitOuterPhase.next) { // outer accessors are entered at explicitOuter + 1, should not be defined before.
-      newSymbol(owner, name, SyntheticArtifact | flags, info, coord = cls.coord)
+      newSymbol(owner, name, SyntheticArtifact | flags, info, coord = cls.coord, nestingLevel = currentNestingLevel)
     }
   }
 

tests/neg/i16343.scala

@@ -0,0 +1,2 @@
+class Issue16343:
+  class MyWorker extends javax.swing.SwingWorker[Unit, Unit] // error