Fix Unit ascriptions in erasure

We should simply drop a Unit ascription, not ascribe BoxedUnit instead. This fixes
a case in `nullAsInstanceOf.scala`. Now

    null.asInstanceOf[Unit]

is `()`, whereas before it was `null` `()` is correct, or at least it
is the same behavior as for all other primitive types.

Author: odersky

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

@@ -588,16 +588,17 @@ object Erasure {
       tree.withType(erasure(tree.tpe))
 
     /** This override is only needed to semi-erase type ascriptions */
-    override def typedTyped(tree: untpd.Typed, pt: Type)(using Context): Tree = {
+    override def typedTyped(tree: untpd.Typed, pt: Type)(using Context): Tree =
       val Typed(expr, tpt) = tree
-      val tpt1 = tpt match {
-        case Block(_, tpt) => tpt // erase type aliases (statements) from type block
-        case tpt => tpt
-      }
-      val tpt2 = typedType(tpt1)
-      val expr1 = typed(expr, tpt2.tpe)
-      assignType(untpd.cpy.Typed(tree)(expr1, tpt2), tpt2)
-    }
+      if tpt.typeOpt.typeSymbol == defn.UnitClass then
+        typed(expr, defn.UnitType)
+      else
+        val tpt1 = tpt match
+          case Block(_, tpt) => tpt // erase type aliases (statements) from type block
+          case tpt => tpt
+        val tpt2 = typedType(tpt1)
+        val expr1 = typed(expr, tpt2.tpe)
+        assignType(untpd.cpy.Typed(tree)(expr1, tpt2), tpt2)
 
     override def typedLiteral(tree: untpd.Literal)(using Context): Tree =
       if (tree.typeOpt.isRef(defn.UnitClass))