Reproduce and fix error that affected http4s

Author: dwijnand

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

@@ -339,7 +339,7 @@ object Implicits:
         def preferDefinitions = isImport && !outer.isImport
         def preferNamedImport = isWildcardImport && !isWildcardImport(using outer.irefCtx)
 
-        if level == outer.level && (preferDefinitions || preferNamedImport) then
+        if !migrateTo3(using irefCtx) && level == outer.level && (preferDefinitions || preferNamedImport) then
           // special cases: definitions beat imports, and named imports beat
           // wildcard imports, provided both are in contexts with same scope
           filter(ownEligible, outerEligible) ::: outerEligible

tests/pos/i18183.migration.scala

@@ -0,0 +1,38 @@
+// scalac: -source:3.0-migration
+
+// A not-fully-minimal reproduction of the CI failure in http4s
+// While implementing the fix to name "shadowing" in implicit lookup.
+
+import scala.util.control.NoStackTrace
+
+final case class EitherT[F[_], A, B](value: F[Either[A, B]]) {
+  def semiflatMap[D](f: B => F[D])(implicit F: Monad[F]): EitherT[F, A, D] = ???
+}
+
+trait Applicative[F[_]] {
+  def pure[A](x: A): F[A]
+}
+trait Monad[F[_]] extends Applicative[F]
+trait Async[F[_]] extends Monad[F]
+
+final class Request[+F[_]]
+
+final case class RequestCookie(name: String, content: String)
+
+final class CSRF2[F[_], G[_]](implicit F: Async[F]) { self =>
+  import CSRF2._
+
+  def signToken[M[_]](rawToken: String)(implicit F: Async[M]): M[CSRFToken] = ???
+
+  def refreshedToken[M[_]](implicit F: Async[M]): EitherT[M, CSRFCheckFailed, CSRFToken] =
+    EitherT(extractRaw("")).semiflatMap(signToken[M])
+
+  def extractRaw[M[_]: Async](rawToken: String): M[Either[CSRFCheckFailed, String]] = ???
+}
+
+object CSRF2 {
+  type CSRFToken
+
+  case object CSRFCheckFailed extends Exception("CSRF Check failed") with NoStackTrace
+  type CSRFCheckFailed = CSRFCheckFailed.type
+}