Verify that constraints do not contain wildcard types

Author: odersky

compiler/src/dotty/tools/dotc/config/Config.scala

@@ -34,6 +34,9 @@ object Config {
    */
   inline val checkConstraintsPropagated = false
 
+  /** Check that constraint bounds do not contain wildcard types */
+  inline val checkNoWildcardsInConstraint = true
+
   /** If a constraint is over a type lambda `tl` and `tvar` is one of
    *  the type variables associated with `tl` in the constraint, check
    *  that the origin of `tvar` is a parameter of `tl`.

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

@@ -270,6 +270,7 @@ trait ConstraintHandling {
       var trackedPolis: List[PolyType] = Nil
       def apply(tp: Type) = tp match
         case tp: WildcardType =>
+          assert(false)
           val poly = PolyType(tpnme.EMPTY :: Nil)(pt => tp.bounds :: Nil, pt => defn.AnyType)
           trackedPolis = poly :: trackedPolis
           poly.paramRefs.head
@@ -309,6 +310,7 @@ trait ConstraintHandling {
       def apply(tp: Type) = mapOver {
         tp match
           case tp: WildcardType =>
+            assert(false)
             pickOneBound(tp.bounds)
           case tp: TypeParamRef if substWildcards.trackedPolis.contains(tp.binder) =>
             pickOneBound(fullBounds(tp))

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

@@ -376,6 +376,7 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
       Nil
 
   private def updateEntry(current: This, param: TypeParamRef, tp: Type)(using Context): This = {
+    if Config.checkNoWildcardsInConstraint then assert(!tp.containsWildcardTypes)
     var current1 = boundsLens.update(this, current, param, tp)
     tp match {
       case TypeBounds(lo, hi) =>

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

@@ -139,7 +139,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
     try topLevelSubType(tp1, tp2)
     finally useNecessaryEither = saved
 
-  override protected def allowWildcards: Boolean = !useNecessaryEither
+  override protected def approximateWildcards: Boolean = useNecessaryEither
 
   def testSubType(tp1: Type, tp2: Type): CompareResult =
     GADTused = false

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

@@ -439,7 +439,7 @@ object Types {
 
     /** Does this type contain wildcard types? */
     final def containsWildcardTypes(using Context) =
-      existsPart(_.isInstanceOf[WildcardType], stopAtStatic = true)
+      existsPart(_.isInstanceOf[WildcardType], stopAtStatic = true, forceLazy = false)
 
 // ----- Higher-order combinators -----------------------------------