Fix #6484: Properly unpickle some Scala 2 type lambdas

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -46,7 +46,11 @@ object Scala2Unpickler {
   /** Convert temp poly type to poly type and leave other types alone. */
   def translateTempPoly(tp: Type)(implicit ctx: Context): Type = tp match {
     case TempPolyType(tparams, restpe) =>
-      (if (tparams.head.owner.isTerm) PolyType else HKTypeLambda)
+      // This check used to read `owner.isTerm` but that wasn't always correct,
+      // I'm not sure `owner.is(Method)` is 100% correct either but it seems to
+      // work better. See the commit message where this change was introduced
+      // for more information.
+      (if (tparams.head.owner.is(Method)) PolyType else HKTypeLambda)
         .fromParams(tparams, restpe)
     case tp => tp
   }

sbt-dotty/sbt-test/scala2-compat/eff/build.sbt

@@ -0,0 +1,4 @@
+scalaVersion := sys.props("plugin.scalaVersion")
+
+libraryDependencies +=
+   ("org.atnos" %% "eff" % "5.4.1").withDottyCompat(scalaVersion.value)

sbt-dotty/sbt-test/scala2-compat/eff/i6484.scala

@@ -0,0 +1,13 @@
+import cats._
+import cats.data._
+import cats.implicits._
+import org.atnos.eff._
+import org.atnos.eff.all._
+import org.atnos.eff.syntax.all._
+
+import scala.language.implicitConversions
+
+object Test {
+  def resolve[T](e: Eff[Fx1[[X] => Kleisli[Id, String, X]], T], g: String): T = 
+    e.runReader[String](g)(Member.Member1[[X] => Kleisli[Id, String, X]]).run
+}

sbt-dotty/sbt-test/scala2-compat/eff/project/plugins.sbt

@@ -0,0 +1 @@
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % sys.props("plugin.version"))

sbt-dotty/sbt-test/scala2-compat/eff/test

@@ -0,0 +1 @@
+> compile