chore: do not patch BoxedUnit anymore

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

@@ -1935,11 +1935,6 @@ class Definitions {
       TypeOps.nestedPairs(tparams.map(_.typeRef)) :: parents.tail
   }
 
-  /** If it is BoxedUnit, remove `java.io.Serializable` from `parents`. */
-  def adjustForBoxedUnit(cls: ClassSymbol, parents: List[Type]): List[Type] =
-    if (isBoxedUnitClass(cls)) parents.filter(_.typeSymbol != JavaSerializableClass)
-    else parents
-
   private val HasProblematicGetClass: Set[Name] = Set(
     tpnme.AnyVal, tpnme.Byte, tpnme.Short, tpnme.Char, tpnme.Int, tpnme.Long, tpnme.Float, tpnme.Double,
     tpnme.Unit, tpnme.Boolean)

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -106,7 +106,7 @@ object Scala2Unpickler {
     // If from Scala 2, adjust for tuple classes; if not, it's from Java, and adjust for BoxedUnit
     val normalizedParents =
       if (fromScala2) defn.adjustForTuple(cls, tparams, parents1)
-      else defn.adjustForBoxedUnit(cls, parents1)
+      else parents1
     for (tparam <- tparams) {
       val tsym = decls.lookup(tparam.name)
       if (tsym.exists) tsym.setFlag(TypeParam)

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -1749,11 +1749,9 @@ class Namer { typer: Typer =>
       denot.info = tempInfo.nn
 
       val parentTypes = defn.adjustForTuple(cls, cls.typeParams,
-        defn.adjustForBoxedUnit(cls,
-          addUsingTraits:
-            val isJava = ctx.isJava
-            ensureFirstIsClass(cls, parents.map(checkedParentType(_, isJava)))
-        )
+        addUsingTraits:
+          val isJava = ctx.isJava
+          ensureFirstIsClass(cls, parents.map(checkedParentType(_, isJava)))
       )
       typr.println(i"completing $denot, parents = $parents%, %, parentTypes = $parentTypes%, %")
 

library/src/scala/runtime/BoxedUnit.java

@@ -12,15 +12,11 @@
 
 package scala.runtime;
 
-
-public final class BoxedUnit implements java.io.Serializable {
-    private static final long serialVersionUID = 8405543498931817370L;
+public final class BoxedUnit {
 
     public final static BoxedUnit UNIT = new BoxedUnit();
 
     public final static Class<Void> TYPE = java.lang.Void.TYPE;
-
-    private Object readResolve() { return UNIT; }
 
     private BoxedUnit() { }