Keep correct kind when preparing constraint in stripParams

Fixes #14760

Author: odersky

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

@@ -248,7 +248,7 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
       NoType
     case tp: TypeBounds =>
       val lo1 = stripParams(tp.lo, todos, !isUpper).orElse(defn.NothingType)
-      val hi1 = stripParams(tp.hi, todos, isUpper).orElse(defn.AnyKindType)
+      val hi1 = stripParams(tp.hi, todos, isUpper).orElse(tp.kind)
       tp.derivedTypeBounds(lo1, hi1)
     case tp: AndType if isUpper =>
       val tp1 = stripParams(tp.tp1, todos, isUpper)

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

@@ -258,6 +258,19 @@ class TypeApplications(val self: Type) extends AnyVal {
     case _ => NoType
   }
 
+  /** The kind of a type is the largest type capturing the parameter shape
+   *  of a type without looking at precise bounds.
+   *    - The kind of single-kinded types is Any
+   *    - A kind like (* -> *) -> * is represented as [X1 <: [X2] =>> Any] =>> Any
+   */
+  def kind(using Context): Type = self.hkResult match
+    case NoType => defn.AnyType
+    case self: TypeRef if self.symbol == defn.AnyKindClass => self
+    case rt =>
+      HKTypeLambda(
+        self.typeParams.map(tparam => TypeBounds.upper(tparam.paramInfo.hiBound.kind)),
+        rt.kind)
+
   /** Do self and other have the same kinds (not counting bounds and variances)?
    *  Note: An any-kinded type "has the same kind" as any other type.
    */

tests/pos/i14760.scala

@@ -0,0 +1,5 @@
+enum SUB[-A, +B]:
+  case Refl[X]() extends SUB[X, X]
+
+def foo[C, A >: C <: C, B](e: SUB[B, A]) = e match
+  case SUB.Refl() =>