deskolemize the type of ValDef

Author: noti0na1

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

@@ -1208,7 +1208,6 @@ object Types {
         if tp.isOverloaded then tp else tp.underlying.widen
       case tp: SingletonType => tp.underlying.widen
       case tp: ExprType => tp.resultType.widen
-      case tp: AndType => tp.derivedAndType(tp.tp1.widen, tp.tp2.widen)
       case tp =>
         val tp1 = tp.stripped
         if tp1 eq tp then tp
@@ -1390,6 +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

@@ -1713,7 +1713,7 @@ class Namer { typer: Typer =>
         }
     }
 
-    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/pos/i13197.scala

@@ -6,7 +6,7 @@ extension [T](x: T | String) inline def forceString: x.type & String =
 trait Bar:
   def b: String | Int
 
-class Foo(a: String = "", b: Any)
+class Foo(a: String = "", b: String)
 
 object Foo:
   def foo(bar: Bar) = Foo(b = bar.b.forceString)