Fix #13197: Deskolemize lifted named arguments

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

@@ -1389,6 +1389,17 @@ object Types {
     /** Like `dealiasKeepAnnots`, but keeps only refining annotations */
     final def dealiasKeepRefiningAnnots(using Context): Type = dealias1(keepIfRefining)
 
+    /** Approximate this type with a type that does not contain skolem types. */
+    final def deskolemized(using Context): Type =
+      val deskolemizer = new ApproximatingTypeMap {
+        def apply(tp: Type) = /*trace(i"deskolemize($tp) at $variance", show = true)*/
+          tp match {
+            case tp: SkolemType => range(defn.NothingType, atVariance(1)(apply(tp.info)))
+            case _ => mapOver(tp)
+          }
+      }
+      deskolemizer(this)
+
     /** The result of normalization using `tryNormalize`, or the type itself if
      *  tryNormlize yields NoType
      */

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

@@ -48,7 +48,7 @@ abstract class Lifter {
     else {
       val name = UniqueName.fresh(prefix)
       // don't instantiate here, as the type params could be further constrained, see tests/pos/pickleinf.scala
-      var liftedType = expr.tpe.widen
+      var liftedType = expr.tpe.widen.deskolemized
       if (liftedFlags.is(Method)) liftedType = ExprType(liftedType)
       val lifted = newSymbol(ctx.owner, name, liftedFlags | Synthetic, liftedType, coord = spanCoord(expr.span))
       defs += liftedDef(lifted, expr)

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

@@ -1704,16 +1704,7 @@ class Namer { typer: Typer =>
     // it would be erased to BoxedUnit.
     def dealiasIfUnit(tp: Type) = if (tp.isRef(defn.UnitClass)) defn.UnitType else tp
 
-    // Approximate a type `tp` with a type that does not contain skolem types.
-    val deskolemize = new ApproximatingTypeMap {
-      def apply(tp: Type) = /*trace(i"deskolemize($tp) at $variance", show = true)*/
-        tp match {
-          case tp: SkolemType => range(defn.NothingType, atVariance(1)(apply(tp.info)))
-          case _ => mapOver(tp)
-        }
-    }
-
-    def cookedRhsType = deskolemize(dealiasIfUnit(rhsType))
+    def cookedRhsType = dealiasIfUnit(rhsType).deskolemized
     def lhsType = fullyDefinedType(cookedRhsType, "right-hand side", mdef.span)
     //if (sym.name.toString == "y") println(i"rhs = $rhsType, cooked = $cookedRhsType")
     if (inherited.exists)

tests/explicit-nulls/pos/i13197.scala

@@ -0,0 +1,7 @@
+trait Bar:
+  def b: String | Null
+
+class Foo(a: String = "", b: String)
+
+object Foo:
+  def foo(bar: Bar) = Foo(b = bar.b.nn)

tests/pos/i13197.scala

@@ -0,0 +1,12 @@
+// this test is similar to explicit-nulls/pos/i13197.scala, but without explicit nulls
+
+extension [T](x: T | String) inline def forceString: x.type & String =
+  x.asInstanceOf
+
+trait Bar:
+  def b: String | Int
+
+class Foo(a: String = "", b: String)
+
+object Foo:
+  def foo(bar: Bar) = Foo(b = bar.b.forceString)