Approximate GADT lower bound for HKTs

Author: Linyxus

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

@@ -1167,13 +1167,23 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         else
           fallback(tycon2bounds.lo)
 
+      def compareTyConGadtBounds: Boolean =
+        tycon2 match
+          case tycon2: TypeRef =>
+            val tycon2sym = tycon2.symbol
+            tycon2sym.onGadtBounds { bounds2 =>
+              compareLower(bounds2, tyconIsTypeRef = false)
+            }
+          case _ => false
+
       tycon2 match {
         case param2: TypeParamRef =>
           isMatchingApply(tp1) ||
           canConstrain(param2) && canInstantiate(param2) ||
           compareLower(bounds(param2), tyconIsTypeRef = false)
         case tycon2: TypeRef =>
           isMatchingApply(tp1) ||
+          compareTyConGadtBounds ||
           defn.isCompiletimeAppliedType(tycon2.symbol) && compareCompiletimeAppliedType(tp2, tp1, fromBelow = true) || {
             tycon2.info match {
               case info2: TypeBounds =>