Fixes IllegalAccessError with Java package protected class (#21362)

This is a backport of Scala 2.x scala/scala#6023 by @lrytz 

Fixes #13841
Fixes #13897

## Problem
When compiling `builder.call1().call2()` where both are Java-defined
package-protected class through a public subsclass, Scala 3 does not
properly cast the receiver to the public class, and results in an
IllegalAccessError.

## Solution
This backports the casting fix from the Scala 2.x compiler.

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

@@ -36,6 +36,7 @@ import ExplicitOuter.*
 import core.Mode
 import util.Property
 import reporting.*
+import scala.annotation.tailrec
 
 class Erasure extends Phase with DenotTransformer {
 
@@ -764,7 +765,8 @@ object Erasure {
           (ctx.owner.enclosingPackageClass eq boundary)
         }
 
-      def recur(qual: Tree): Tree = {
+      @tailrec
+      def recur(qual: Tree): Tree =
         val qualIsPrimitive = qual.tpe.widen.isPrimitiveValueType
         val symIsPrimitive = sym.owner.isPrimitiveValueClass
 
@@ -773,33 +775,34 @@ object Erasure {
             inContext(preErasureCtx):
               tree.qualifier.typeOpt.widen.finalResultType)
 
-        if (qualIsPrimitive && !symIsPrimitive || qual.tpe.widenDealias.isErasedValueType)
+        if qualIsPrimitive && !symIsPrimitive || qual.tpe.widenDealias.isErasedValueType then
           recur(box(qual))
-        else if (!qualIsPrimitive && symIsPrimitive)
+        else if !qualIsPrimitive && symIsPrimitive then
           recur(unbox(qual, sym.owner.typeRef))
-        else if (sym.owner eq defn.ArrayClass)
+        else if sym.owner eq defn.ArrayClass then
           selectArrayMember(qual, originalQual)
-        else {
-          val qual1 = adaptIfSuper(qual)
-          if (qual1.tpe.derivesFrom(sym.owner) || qual1.isInstanceOf[Super])
-            select(qual1, sym)
-          else
-            val castTarget = // Avoid inaccessible cast targets, see i8661
-              if isJvmAccessible(sym.owner) && sym.owner.isType
-              then
-                sym.owner.typeRef
-              else
-                // If the owner is inaccessible, try going through the qualifier,
-                // but be careful to not go in an infinite loop in case that doesn't
-                // work either.
-                val tp = originalQual
-                if tp =:= qual1.tpe.widen then
-                  return errorTree(qual1,
-                    em"Unable to emit reference to ${sym.showLocated}, ${sym.owner} is not accessible in ${ctx.owner.enclosingClass}")
-                tp
-            recur(cast(qual1, castTarget))
-        }
-      }
+        else
+          adaptIfSuper(qual) match
+            case qual1: Super =>
+              select(qual1, sym)
+            case qual1 if !isJvmAccessible(qual1.tpe.typeSymbol)
+                || !qual1.tpe.derivesFrom(sym.owner) =>
+              val castTarget = // Avoid inaccessible cast targets, see i8661
+                if isJvmAccessible(sym.owner) && sym.owner.isType then
+                  sym.owner.typeRef
+                else
+                  // If the owner is inaccessible, try going through the qualifier,
+                  // but be careful to not go in an infinite loop in case that doesn't
+                  // work either.
+                  val tp = originalQual
+                  if tp =:= qual1.tpe.widen then
+                    return errorTree(qual1,
+                      em"Unable to emit reference to ${sym.showLocated}, ${sym.owner} is not accessible in ${ctx.owner.enclosingClass}")
+                  tp
+              recur(cast(qual1, castTarget))
+            case qual1 =>
+              select(qual1, sym)
+      end recur
 
       checkNotErased(recur(qual1))
     }

tests/run/java-package-protected/A.java

@@ -0,0 +1,21 @@
+// filter: unchecked
+package a;
+
+/** This is package protected. */
+class B<T extends B<T>> {
+    private int connectTimeout = 10000;
+    private int failedAttempts = 3;
+
+    public T setConnectTimeout(int connectTimeout) {
+        this.connectTimeout = connectTimeout;
+        return (T) this;
+    }
+
+    public T setFailedAttempts(int failedAttempts) {
+        this.failedAttempts = failedAttempts;
+        return (T) this;
+    }
+}
+
+/** This is public. */
+public class A extends B<A> { }

tests/run/java-package-protected/C.scala

@@ -0,0 +1,10 @@
+package b
+
+import a.*
+
+object C:
+  def m: Int =
+    val a = new A()
+      .setConnectTimeout(1)
+      .setFailedAttempts(1)
+    0

tests/run/java-package-protected/Test.scala

@@ -0,0 +1,5 @@
+// scalajs: --skip
+
+object Test extends App:
+  assert(b.C.m == 0)
+end Test