Disable TypeApplications#translateToRepeated distribution of AndTypes

Author: EugeneFlesselle

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

@@ -475,13 +475,19 @@ class TypeApplications(val self: Type) extends AnyVal {
       self.derivedExprType(tp.translateParameterized(from, to))
     case _ =>
       if (self.derivesFrom(from)) {
+        // NOTE: we assume the `To` class is covariant s.t.
+        // `To[T] X To[U] <:< To[T | U]` where X ::= `&` | `|`
         def elemType(tp: Type): Type = tp.widenDealias match
           case tp: OrType =>
             if tp.tp1.isBottomType then elemType(tp.tp2)
             else if tp.tp2.isBottomType then elemType(tp.tp1)
             else tp.derivedOrType(elemType(tp.tp1), elemType(tp.tp2))
-          case tp: AndType => tp.derivedAndType(elemType(tp.tp1), elemType(tp.tp2))
-          case _ => tp.baseType(from).argInfos.headOption.getOrElse(defn.NothingType)
+          case AndType(tp1, tp2) =>
+            // see #23435 for why this is not `tp.derivedAndType(elemType(tp1), ...)`
+            OrType(elemType(tp1), elemType(tp2), soft = false)
+          case _ =>
+            tp.baseType(from).argInfos.headOption.getOrElse(defn.NothingType)
+        end elemType
         val arg = elemType(self)
         val arg1 = if (wildcardArg) TypeBounds.upper(arg) else arg
         to.typeRef.appliedTo(arg1)

tests/pos/i8056.scala

@@ -1,5 +1,8 @@
-object O{
+object O {
   def m(x: Any*) = ()
-  def n(l: List[Int] | List[String]): Unit = m(l*)
-  def n2(l: List[Int] & List[String]): Unit = m(l*)
+
+  def n2(l: List[Int] | List[String]): Unit = m(l)
+  def n1(l: List[Int] | List[String]): Unit = m(l*)
+  def m2(l: List[Int] & List[String]): Unit = m(l)
+  def m1(l: List[Int] & List[String]): Unit = m(l*)
 }