Merge pull request #9140 from dotty-staging/fix-9044

Quickly fix HKT GADT issue

Author: abgruszecki

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

@@ -3398,34 +3398,10 @@ class Typer extends Namer
 
       // try an implicit conversion
       val prevConstraint = ctx.typerState.constraint
-      def recover(failure: SearchFailureType) = {
-        def canTryGADTHealing: Boolean = {
-          val isDummy = tree.hasAttachment(dummyTreeOfType.IsDummyTree)
-          tryGadtHealing // allow GADT healing only once to avoid a loop
-          && ctx.gadt.nonEmpty // GADT healing only makes sense if there are GADT constraints present
-          && !isDummy // avoid healing a dummy tree as it can lead to an error in a very specific case
-        }
-
+      def recover(failure: SearchFailureType) =
         if (isFullyDefined(wtp, force = ForceDegree.all) &&
             ctx.typerState.constraint.ne(prevConstraint)) readapt(tree)
-        else if (canTryGADTHealing) {
-          // try recovering with a GADT approximation
-          val nestedCtx = ctx.fresh.setNewTyperState()
-          val res =
-            readapt(
-              tree = tpd.Typed(tree, TypeTree(Inferencing.approximateGADT(wtp))),
-              shouldTryGadtHealing = false,
-            )(using nestedCtx)
-          if (!nestedCtx.reporter.hasErrors) {
-            // GADT recovery successful
-            nestedCtx.typerState.commit()
-            res
-          } else {
-            // otherwise fail with the error that would have been reported without the GADT recovery
-            err.typeMismatch(tree, pt, failure)
-          }
-        } else err.typeMismatch(tree, pt, failure)
-      }
+        else err.typeMismatch(tree, pt, failure)
       if ctx.mode.is(Mode.ImplicitsEnabled) && tree.typeOpt.isValueType then
         if pt.isRef(defn.AnyValClass) || pt.isRef(defn.ObjectClass) then
           ctx.error(em"the result of an implicit conversion must be more specific than $pt", tree.sourcePos)

tests/neg/i9044.scala

@@ -0,0 +1,14 @@
+sealed trait Test[+F[_], +A] extends Product with Serializable
+
+object Test {
+
+  implicit class Syntax[F[_], A](val self: Test[F, A]) extends AnyVal {
+
+    def fold[B](completed: F[A] => B): B = self match {
+      case Completed(fa) => completed(fa) // error
+    }
+  }
+
+
+  final case class Completed[F[_], A](fa: F[A]) extends Test[F, A]
+}