Select local implicits over name-imported over wildcard imported

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

@@ -299,7 +299,7 @@ object Implicits:
   class ContextualImplicits(
       val refs: List[ImplicitRef],
       val outerImplicits: ContextualImplicits | Null,
-      isImport: Boolean)(initctx: Context) extends ImplicitRefs(initctx) {
+      val isImport: Boolean)(initctx: Context) extends ImplicitRefs(initctx) {
     private val eligibleCache = EqHashMap[Type, List[Candidate]]()
 
     /** The level increases if current context has a different owner or scope than
@@ -330,8 +330,21 @@ object Implicits:
       if ownEligible.isEmpty then outerEligible
       else if outerEligible.isEmpty then ownEligible
       else
-        val shadowed = ownEligible.map(_.ref.implicitName).toSet
-        ownEligible ::: outerEligible.filterConserve(cand => !shadowed.contains(cand.ref.implicitName))
+        def filter(xs: List[Candidate], remove: List[Candidate]) =
+          val shadowed = remove.map(_.ref.implicitName).toSet
+          xs.filterConserve(cand => !shadowed.contains(cand.ref.implicitName))
+
+        val outer = outerImplicits.uncheckedNN
+        def isWildcardImport(using Context) = ctx.importInfo.nn.isWildcardImport
+        def preferDefinitions = isImport && !outer.isImport
+        def preferNamedImport = isWildcardImport && !isWildcardImport(using outer.irefCtx)
+
+        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
+        else
+          ownEligible ::: filter(outerEligible, ownEligible)
 
     def uncachedEligible(tp: Type)(using Context): List[Candidate] =
       Stats.record("uncached eligible")

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
+}

tests/run/i18183.findRef.scala

@@ -0,0 +1,17 @@
+// A minimised reproduction of how an initial change to combineEligibles broke Typer#findRef
+case class Foo(n: Int)
+
+class Test:
+  import this.toString
+
+  val foo1 = Foo(1)
+  val foo2 = Foo(2)
+
+  def foo(using Foo): Foo =
+    import this.*
+    def bar(using Foo): Foo = summon[Foo]
+    bar(using foo2)
+
+object Test extends Test:
+  def main(args: Array[String]): Unit =
+    assert(foo(using foo1) eq foo2)

tests/run/i18183.given.scala

@@ -0,0 +1,93 @@
+case class Foo(n: Int)
+
+class Bar(n: Int):
+  given foo: Foo = new Foo(n)
+
+class InMethod:
+  def wild(bar: Bar): Unit =
+    import bar.*
+    given foo: Foo = new Foo(2)
+    assert(foo eq summon[Foo])
+
+  def givenWild(bar: Bar): Unit =
+    import bar.{ given, * }
+    given foo: Foo = new Foo(2)
+    assert(foo eq summon[Foo])
+
+  def givn(bar: Bar): Unit =
+    import bar.given
+    given foo: Foo = new Foo(2)
+    assert(foo eq summon[Foo])
+
+  def givenFoo(bar: Bar): Unit =
+    import bar.given Foo
+    given foo: Foo = new Foo(2)
+    assert(foo eq summon[Foo])
+
+  def named(bar: Bar): Unit =
+    import bar.foo
+    given foo: Foo = new Foo(2)
+    assert(foo eq summon[Foo])
+
+  def namedGivenWild(bar: Bar, bar2: Bar): Unit =
+    import bar.foo, bar2.{ given, * }
+    assert(bar.foo eq summon[Foo])
+
+  def givenWildNamed(bar: Bar, bar2: Bar): Unit =
+    import bar2.{ given, * }, bar.foo
+    assert(bar.foo eq summon[Foo])
+
+  def namedWild(bar: Bar, bar2: Bar): Unit =
+    import bar.foo, bar2.*
+    assert(bar.foo eq summon[Foo])
+
+  def wildNamed(bar: Bar, bar2: Bar): Unit =
+    import bar2.*, bar.foo
+    assert(bar.foo eq summon[Foo])
+
+class InClassWild(bar: Bar):
+  import bar.*
+  given foo: Foo = new Foo(2)
+  assert(foo eq summon[Foo])
+
+class InClassGivenWild(bar: Bar):
+  import bar.{ given, * }
+  given foo: Foo = new Foo(2)
+  assert(foo eq summon[Foo])
+
+class InClassGiven(bar: Bar):
+  import bar.given
+  given foo: Foo = new Foo(2)
+  assert(foo eq summon[Foo])
+
+class InClassGivenFoo(bar: Bar):
+  import bar.given Foo
+  given foo: Foo = new Foo(2)
+  assert(foo eq summon[Foo])
+
+class InClassNamed(bar: Bar):
+  import bar.foo
+  given foo: Foo = new Foo(2)
+  assert(foo eq summon[Foo])
+
+object Test:
+  def main(args: Array[String]): Unit =
+    val bar = new Bar(1)
+    val bar2 = new Bar(2)
+
+    new InMethod().wild(bar)
+    new InMethod().givenWild(bar)            // was: error
+    new InMethod().givn(bar)                 // was: error
+    new InMethod().givenFoo(bar)             // was: error
+    new InMethod().named(bar)                // was: error
+
+    new InMethod().namedWild(bar, bar2)
+    new InMethod().wildNamed(bar, bar2)
+    new InMethod().namedGivenWild(bar, bar2) // was: error
+    new InMethod().givenWildNamed(bar, bar2)
+
+    new InClassWild(bar)
+    new InClassGivenWild(bar)                // was: error
+    new InClassGiven(bar)                    // was: error
+    new InClassGivenFoo(bar)                 // was: error
+    new InClassNamed(bar)                    // was: error

tests/run/i18183.mixed.scala

@@ -0,0 +1,141 @@
+case class Foo(n: Int)
+
+class OldBar(n: Int):
+  implicit val foo: Foo = new Foo(n)
+
+class NewBar(n: Int):
+  given foo: Foo = new Foo(n)
+
+class OldInMethod:
+  def wild(bar: OldBar): Unit =
+    import bar.*
+    given foo: Foo = new Foo(2)
+    assert(foo eq summon[Foo])
+
+  def named(bar: OldBar): Unit =
+    import bar.foo
+    given foo: Foo = new Foo(2)
+    assert(foo eq summon[Foo])
+
+  def namedWild(bar: OldBar, bar2: NewBar): Unit =
+    import bar.foo, bar2.*
+    assert(bar.foo eq summon[Foo])
+
+  def wildNamed(bar: OldBar, bar2: NewBar): Unit =
+    import bar2.*, bar.foo
+    assert(bar.foo eq summon[Foo])
+
+  def namedGivenWild(bar: OldBar, bar2: NewBar): Unit =
+    import bar.foo
+    import bar2.{ given, * }
+    assert(bar.foo eq summon[Foo])
+
+  def givenWildNamed(bar: OldBar, bar2: NewBar): Unit =
+    import bar2.{ given, * }, bar.foo
+    assert(bar.foo eq summon[Foo])
+
+class OldInClassWild(bar: OldBar):
+  import bar.*
+  given foo: Foo = new Foo(2)
+  assert(foo eq summon[Foo])
+
+class OldInClassNamed(bar: OldBar):
+  import bar.foo
+  given foo: Foo = new Foo(2)
+  assert(foo eq summon[Foo])
+
+
+class NewInMethod:
+  def givenWild(bar: NewBar): Unit =
+    import bar.{ given, * }
+    implicit val foo: Foo = new Foo(2)
+    assert(foo eq summon[Foo])
+
+  def wild(bar: NewBar): Unit =
+    import bar.*
+    implicit val foo: Foo = new Foo(2)
+    assert(foo eq summon[Foo])
+
+  def givn(bar: NewBar): Unit =
+    import bar.given
+    implicit val foo: Foo = new Foo(2)
+    assert(foo eq summon[Foo])
+
+  def givenFoo(bar: NewBar): Unit =
+    import bar.given Foo
+    implicit val foo: Foo = new Foo(2)
+    assert(foo eq summon[Foo])
+
+  def named(bar: NewBar): Unit =
+    import bar.foo
+    implicit val foo: Foo = new Foo(2)
+    assert(foo eq summon[Foo])
+
+  def namedWild(bar: NewBar, bar2: OldBar): Unit =
+    import bar.foo, bar2.*
+    assert(bar.foo eq summon[Foo])
+
+  def wildNamed(bar: NewBar, bar2: OldBar): Unit =
+    import bar2.*, bar.foo
+    assert(bar.foo eq summon[Foo])
+
+class NewInClassGivenWild(bar: NewBar):
+  import bar.{ given, * }
+  implicit val foo: Foo = new Foo(2)
+  assert(foo eq summon[Foo])
+
+class NewInClassWild(bar: NewBar):
+  import bar.*
+  implicit val foo: Foo = new Foo(2)
+  assert(foo eq summon[Foo])
+
+class NewInClassGiven(bar: NewBar):
+  import bar.given
+  implicit val foo: Foo = new Foo(2)
+  assert(foo eq summon[Foo])
+
+class NewInClassGivenFoo(bar: NewBar):
+  import bar.given Foo
+  implicit val foo: Foo = new Foo(2)
+  assert(foo eq summon[Foo])
+
+class NewInClassNamed(bar: NewBar):
+  import bar.foo
+  implicit val foo: Foo = new Foo(2)
+  assert(foo eq summon[Foo])
+
+
+object Test:
+  def main(args: Array[String]): Unit =
+    val oldBar = new OldBar(1)
+    val newBar = new NewBar(1)
+    val oldBar2 = new OldBar(2)
+    val newBar2 = new NewBar(2)
+
+
+    new OldInMethod().wild(oldBar)                    // was: error
+    new OldInMethod().named(oldBar)                   // was: error
+
+    new OldInMethod().namedWild(oldBar, newBar2)
+    new OldInMethod().wildNamed(oldBar, newBar2)
+    new OldInMethod().namedGivenWild(oldBar, newBar2) // was: error
+    new OldInMethod().givenWildNamed(oldBar, newBar2)
+
+    new OldInClassWild(oldBar)                        // was: error
+    new OldInClassNamed(oldBar)                       // was: error
+
+
+    new NewInMethod().wild(newBar)
+    new NewInMethod().givenWild(newBar)               // was: error
+    new NewInMethod().givn(newBar)                    // was: error
+    new NewInMethod().givenFoo(newBar)                // was: error
+    new NewInMethod().named(newBar)                   // was: error
+
+    new NewInMethod().namedWild(newBar, oldBar2)      // was: error
+    new NewInMethod().wildNamed(newBar, oldBar2)
+
+    new NewInClassWild(newBar)
+    new NewInClassGivenWild(newBar)                   // was: error
+    new NewInClassGiven(newBar)                       // was: error
+    new NewInClassGivenFoo(newBar)                    // was: error
+    new NewInClassNamed(newBar)                       // was: error

tests/run/i18183.scala

@@ -0,0 +1,49 @@
+case class Foo(n: Int)
+
+class Bar(n: Int):
+  implicit val foo: Foo = new Foo(n)
+
+class InMethod:
+  def wild(bar: Bar): Unit =
+    import bar._
+    implicit val foo: Foo = new Foo(2)
+    assert(foo eq implicitly[Foo])
+
+  def named(bar: Bar): Unit =
+    import bar.foo
+    implicit val foo: Foo = new Foo(2)
+    assert(foo eq implicitly[Foo])
+
+  def namedWild(bar: Bar, bar2: Bar): Unit =
+    import bar.foo
+    import bar2._
+    assert(bar.foo eq implicitly[Foo])
+
+  def wildNamed(bar: Bar, bar2: Bar): Unit =
+    import bar2._
+    import bar.foo
+    assert(bar.foo eq implicitly[Foo])
+
+class InClassWild(bar: Bar):
+  import bar._
+  implicit val foo: Foo = new Foo(2)
+  assert(foo eq implicitly[Foo])
+
+class InClassNamed(bar: Bar):
+  import bar.foo
+  implicit val foo: Foo = new Foo(2)
+  assert(foo eq implicitly[Foo])
+
+object Test:
+  def main(args: Array[String]): Unit =
+    val bar = new Bar(1)
+    val bar2 = new Bar(2)
+
+    new InMethod().wild(bar)            // was: error
+    new InMethod().named(bar)           // was: error
+
+    new InMethod().namedWild(bar, bar2) // was: error
+    new InMethod().wildNamed(bar, bar2)
+
+    new InClassWild(bar)                // was: error
+    new InClassNamed(bar)               // was: error