(Re-)Drop inaccessible subclasses from refineUsingParent (#21930)

Re-fixes #21790
Closes #21896

Author: hamzaremmal

compiler/src/dotty/tools/dotc/core/Decorators.scala

@@ -292,7 +292,7 @@ object Decorators {
       case _ => String.valueOf(x).nn
 
     /** Returns the simple class name of `x`. */
-    def className: String = x.getClass.getSimpleName.nn
+    def className: String = if x == null then "<null>" else x.getClass.getSimpleName.nn
 
   extension [T](x: T)
     def assertingErrorsReported(using Context): T = {

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -900,7 +900,11 @@ object TypeOps:
       for tp <- mixins.reverseIterator do
         protoTp1 <:< tp
       maximizeType(protoTp1, NoSpan)
-      wildApprox(protoTp1)
+      val inst = wildApprox(protoTp1)
+      if inst.classSymbols.isEmpty then
+        // E.g. i21790, can't instantiate S#CA as a subtype of O.A, because O.CA isn't accessible
+        NoType
+      else inst
     }
 
     if (protoTp1 <:< tp2) instantiate()

tests/pos/i21790.scala

@@ -0,0 +1,14 @@
+package p
+
+trait S:
+  sealed trait A
+  private class CA() extends A
+
+object O extends S
+
+trait T
+
+class Test:
+  def f(e: T) = e match
+    case _: O.A =>
+    case _      =>