Fix inter-dependencies in HK GADTs

Author: abgruszecki

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

@@ -124,7 +124,7 @@ final class ProperGadtConstraint private(
 
     // The replaced symbols are picked up here.
     addToConstraint(poly1, tvars)
-      .reporting(i"added to constraint: $params%, %\n$debugBoundsDescription", gadts)
+      .reporting(i"added to constraint: [$poly1] $params%, %\n$debugBoundsDescription", gadts)
   }
 
   override def addBound(sym: Symbol, bound: Type, isUpper: Boolean)(implicit ctx: Context): Boolean = {
@@ -237,8 +237,11 @@ final class ProperGadtConstraint private(
      }
 
    override def fullUpperBound(param: TypeParamRef)(implicit ctx: Context): Type =
-     constraint.minUpper(param).foldLeft(nonParamBounds(param).hi) {
-       (t, u) => t & externalize(u)
+     constraint.minUpper(param).foldLeft(nonParamBounds(param).hi) { (t, u) =>
+       val eu = externalize(u)
+       // Any as the upper bound means "no bound", but if F is higher-kinded,
+       // Any & F = F[_]; this is wrong for us so we need to short-circuit
+       if t.isAny then eu else t & eu
      }
 
   // ---- Private ----------------------------------------------------------

tests/pos/gadt-hk-ordering.scala

@@ -0,0 +1,21 @@
+object test {
+
+  enum KSUB[-F[_], +G[_]] {
+    case Refl[S[_]]() extends KSUB[S, S]
+  }
+
+  enum Thing[+F[_]] {
+    case LBOption() extends Thing[Option];
+    case
+  }
+
+  def foo[F[_]](ksub: Option KSUB F) =
+    ksub match {
+      case KSUB.Refl() =>
+        // we have (s is type parameter of KSUB.Refl):
+        // f >: Option
+        // s <: f
+        val fi: F[Int] = Option(0)
+        ()
+    }
+}