Don't allow wildcard types in constraints

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 = false
+
   /** 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

@@ -10,6 +10,7 @@ import Flags._
 import config.Config
 import config.Printers.typr
 import reporting.trace
+import typer.ProtoTypes.newTypeVar
 import StdNames.tpnme
 
 /** Methods for adding constraints and solving them.
@@ -78,22 +79,29 @@ trait ConstraintHandling {
   def fullBounds(param: TypeParamRef)(using Context): TypeBounds =
     nonParamBounds(param).derivedTypeBounds(fullLowerBound(param), fullUpperBound(param))
 
-  protected def addOneBound(param: TypeParamRef, bound: Type, isUpper: Boolean)(using Context): Boolean =
+  /** If true, eliminate wildcards in bounds by avoidance, otherwise replace
+   *  them by fresh variables.
+   */
+  protected def approximateWildcards: Boolean = true
+
+  protected def addOneBound(param: TypeParamRef, rawBound: Type, isUpper: Boolean)(using Context): Boolean =
     if !constraint.contains(param) then true
-    else if !isUpper && param.occursIn(bound) then
+    else if !isUpper && param.occursIn(rawBound) then
       // We don't allow recursive lower bounds when defining a type,
       // so we shouldn't allow them as constraints either.
       false
     else
+      val dropWildcards = new AvoidWildcardsMap:
+        if !isUpper then variance = -1
+        override def mapWild(t: WildcardType) =
+          if approximateWildcards then super.mapWild(t)
+          else newTypeVar(apply(t.effectiveBounds).toBounds)
+      val bound = dropWildcards(rawBound)
       val oldBounds @ TypeBounds(lo, hi) = constraint.nonParamBounds(param)
       val equalBounds = (if isUpper then lo else hi) eq bound
-      if equalBounds
-        && !bound.existsPart(bp => bp.isInstanceOf[WildcardType] || (bp eq param))
-      then
-        // The narrowed bounds are equal and do not contain wildcards,
+      if equalBounds && !bound.existsPart(_ eq param, stopAtStatic = true) then
+        // The narrowed bounds are equal and not recursive,
         // so we can remove `param` from the constraint.
-        // (Handling wildcards requires choosing a bound, but we don't know which
-        // bound to choose here, this is handled in `ConstraintHandling#approximation`)
         constraint = constraint.replace(param, bound)
         true
       else
@@ -245,81 +253,11 @@ trait ConstraintHandling {
    *  @pre `param` is in the constraint's domain.
    */
   final def approximation(param: TypeParamRef, fromBelow: Boolean)(using Context): Type =
-
-    /** Substitute wildcards with fresh TypeParamRefs, to be compared with
-     *  other bound, so that they can be instantiated.
-     */
-    object substWildcards extends TypeMap:
-      override def stopAtStatic = true
-
-      var trackedPolis: List[PolyType] = Nil
-      def apply(tp: Type) = tp match
-        case tp: WildcardType =>
-          val poly = PolyType(tpnme.EMPTY :: Nil)(pt => tp.bounds :: Nil, pt => defn.AnyType)
-          trackedPolis = poly :: trackedPolis
-          poly.paramRefs.head
-        case _ =>
-          mapOver(tp)
-    end substWildcards
-
-    /** Replace TypeParamRefs substituted for wildcards by `substWildCards`
-     *  and any remaining wildcards by a safe approximation
-     */
-    val replaceWildcards = new TypeMap:
-      override def stopAtStatic = true
-
-      /** Try to instantiate a wildcard or TypeParamRef representing a wildcard
-       *  to a type that is known to conform to it.
-       *  This means:
-       *  If fromBelow is true, we minimize the type overall
-       *  Hence, if variance < 0, pick the maximal safe type: bounds.lo
-       *  (i.e. the whole bounds range is over the type).
-       *  If variance > 0, pick the minimal safe type: bounds.hi
-       *  (i.e. the whole bounds range is under the type).
-       *  If variance == 0, pick bounds.lo anyway (this is arbitrary but in line with
-       *  the principle that we pick the smaller type when in doubt).
-       *  If fromBelow is false, we maximize the type overall and reverse the bounds
-       *  If variance != 0. For variance == 0, we still minimize.
-       *  In summary we pick the bound given by this table:
-       *
-       *  variance    | -1  0   1
-       *  ------------------------
-       *  from below  | lo  lo  hi
-       *  from above  | hi  lo  lo
-       */
-      def pickOneBound(bounds: TypeBounds) =
-        if variance == 0 || fromBelow == (variance < 0) then bounds.lo
-        else bounds.hi
-
-      def apply(tp: Type) = mapOver {
-        tp match
-          case tp: WildcardType =>
-            pickOneBound(tp.bounds)
-          case tp: TypeParamRef if substWildcards.trackedPolis.contains(tp.binder) =>
-            pickOneBound(fullBounds(tp))
-          case _ => tp
-      }
-    end replaceWildcards
-
     constraint.entry(param) match
       case entry: TypeBounds =>
         val useLowerBound = fromBelow || param.occursIn(entry.hi)
-        val rawBound = if useLowerBound then fullLowerBound(param) else fullUpperBound(param)
-        val bound = substWildcards(rawBound)
-        val inst =
-          if bound eq rawBound then bound
-          else
-            // Get rid of wildcards by mapping them to fresh TypeParamRefs
-            // with constraints derived from comparing both bounds, and then
-            // instantiating. See pos/i10161.scala for a test where this matters.
-            val saved = constraint
-            try
-              for poly <- substWildcards.trackedPolis do addToConstraint(poly, Nil)
-              if useLowerBound then bound <:< fullUpperBound(param)
-              else fullLowerBound(param) <:< bound
-              replaceWildcards(bound)
-            finally constraint = saved
-        typr.println(s"approx ${param.show}, from below = $fromBelow, bound = ${bound.show}, inst = ${inst.show}")
+        val inst = if useLowerBound then fullLowerBound(param) else fullUpperBound(param)
+        typr.println(s"approx ${param.show}, from below = $fromBelow, inst = ${inst.show}")
         inst
       case inst =>
         assert(inst.exists, i"param = $param\nconstraint = $constraint")

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

@@ -280,9 +280,11 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
     var current = this
     val todos = new mutable.ListBuffer[(OrderingConstraint, TypeParamRef) => OrderingConstraint]
     var i = 0
+    val dropWildcards = AvoidWildcardsMap()
     while (i < poly.paramNames.length) {
       val param = poly.paramRefs(i)
-      val stripped = stripParams(nonParamBounds(param), todos, isUpper = true)
+      val bounds = dropWildcards(nonParamBounds(param))
+      val stripped = stripParams(bounds, todos, isUpper = true)
       current = updateEntry(current, param, stripped)
       while todos.nonEmpty do
         current = todos.head(current, param)
@@ -376,6 +378,7 @@ 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

@@ -139,6 +139,16 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
     try topLevelSubType(tp1, tp2)
     finally useNecessaryEither = saved
 
+  /** Use avoidance to get rid of wildcards in constraint bounds if
+   *  we are doing a necessary comparison, or the mode is TypeVarsMissContext.
+   *  The idea is that under either of these conditions we are not interested
+   *  in creating a fresh type variable to replace the wildcard. I verified
+   *  that several tests break if one or the other part of the disjunction is dropped.
+   *  (for instance, i12677.scala demands `useNecessaryEither` in the condition)
+   */
+  override protected def approximateWildcards: Boolean =
+    useNecessaryEither || ctx.mode.is(Mode.TypevarsMissContext)
+
   def testSubType(tp1: Type, tp2: Type): CompareResult =
     GADTused = false
     if !topLevelSubType(tp1, tp2) then CompareResult.Fail

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 -----------------------------------
 
@@ -5053,6 +5053,11 @@ object Types {
 
   /** Wildcard type, possibly with bounds */
   abstract case class WildcardType(optBounds: Type) extends CachedGroundType with TermType {
+
+    def effectiveBounds(using Context): TypeBounds = optBounds match
+      case bounds: TypeBounds => bounds
+      case _ => TypeBounds.empty
+
     def derivedWildcardType(optBounds: Type)(using Context): WildcardType =
       if (optBounds eq this.optBounds) this
       else if (!optBounds.exists) WildcardType
@@ -5696,6 +5701,15 @@ object Types {
       lo.toText(printer) ~ ".." ~ hi.toText(printer)
   }
 
+  /** Approximate wildcards by their bounds */
+  class AvoidWildcardsMap(using Context) extends ApproximatingTypeMap:
+    protected def mapWild(t: WildcardType) =
+      val bounds = t.effectiveBounds
+      range(atVariance(-variance)(apply(bounds.lo)), apply(bounds.hi))
+    def apply(t: Type): Type = t match
+      case t: WildcardType => mapWild(t)
+      case _ => mapOver(t)
+
   // ----- TypeAccumulators ----------------------------------------------------
 
   abstract class TypeAccumulator[T](implicit protected val accCtx: Context)

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

@@ -707,8 +707,10 @@ object ProtoTypes {
         if wildcardOnly
            || ctx.mode.is(Mode.TypevarsMissContext)
            || !ref.underlying.widenExpr.isValueTypeOrWildcard
-        then WildcardType
-        else newDepTypeVar(ref)
+        then
+          WildcardType(ref.underlying.substParams(mt, mt.paramRefs.map(_ => WildcardType)).toBounds)
+        else
+          newDepTypeVar(ref)
       mt.resultType.substParams(mt, mt.paramRefs.map(replacement))
     else mt.resultType
 

tests/pos/i12677.scala

@@ -0,0 +1,31 @@
+class F[A]
+object F {
+  def apply[A](a: => A) = new F[A]
+}
+
+trait TC[A] { type Out }
+object TC {
+  implicit def tc[A]: TC[A] { type Out = String } = ???
+}
+
+// ====================================================================================
+object Bug {
+  final class CustomHook[A] {
+    def blah(implicit tc: TC[A]): CustomHook[tc.Out] = ???
+  }
+
+  def i: CustomHook[Int] = ???
+  val f = F(i.blah)
+  f: F[CustomHook[String]] // error
+}
+
+// ====================================================================================
+object Workaround {
+  final class CustomHook[A] {
+    def blah[B](implicit tc: TC[A] { type Out = B }): CustomHook[B] = ??? // raise type
+  }
+
+  def i: CustomHook[Int] = ???
+  val f = F(i.blah)
+  f: F[CustomHook[String]] // works
+}