Modify type erasure logic of nullable unions for Scala 2 pseudo-unions (#24130)

Author: noti0na1

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

@@ -683,7 +683,15 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
           if e1.isInstanceOf[WildcardType] || e2.isInstanceOf[WildcardType] then WildcardType
           else erasedGlb(e1, e2)
       case OrType(tp1, tp2) =>
-        if isSymbol && sourceLanguage.isScala2 && ctx.settings.scalajs.value then
+        val e1 = this(tp1)
+        val e2 = this(tp2)
+        val result = if e1.isInstanceOf[WildcardType] || e2.isInstanceOf[WildcardType]
+          then WildcardType
+          else TypeComparer.orType(e1, e2, isErased = true)
+        def isNullStripped =
+          tp2.isNullType && e1.derivesFrom(defn.ObjectClass)
+          || tp1.isNullType && e2.derivesFrom(defn.ObjectClass)
+        if isSymbol && sourceLanguage.isScala2 && ctx.settings.scalajs.value && !isNullStripped then
           // In Scala2Unpickler we unpickle Scala.js pseudo-unions as if they were
           // real unions, but we must still erase them as Scala 2 would to emit
           // the correct signatures in SJSIR.
@@ -694,11 +702,7 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
           // impact on overriding relationships so it's best to leave them
           // alone (and this doesn't impact the SJSIR we generate).
           JSDefinitions.jsdefn.PseudoUnionType
-        else
-          val e1 = this(tp1)
-          val e2 = this(tp2)
-          if e1.isInstanceOf[WildcardType] || e2.isInstanceOf[WildcardType] then WildcardType
-          else TypeComparer.orType(e1, e2, isErased = true)
+        else result
       case tp: MethodType =>
         def paramErasure(tpToErase: Type) =
           erasureFn(sourceLanguage, semiEraseVCs, isConstructor, isSymbol, inSigName = false)(tpToErase)