Merge pull request #3592 from dotty-staging/fix-aux-pattern

Fix #3590: look for implicits in WhilcardType

Author: odersky

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

@@ -409,7 +409,7 @@ trait ImplicitRunInfo { self: RunInfo =>
         case tp: AppliedType if !tp.tycon.typeSymbol.isClass =>
           def applyArg(arg: Type) = arg match {
             case TypeBounds(lo, hi) => AndType.make(lo, hi)
-            case _: WildcardType => defn.AnyType
+            case WildcardType(TypeBounds(lo, hi)) => AndType.make(lo, hi)
             case _ => arg
           }
           (apply(tp.tycon) /: tp.args)((tc, arg) => AndType.make(tc, applyArg(arg)))

tests/pos/i3590.scala

@@ -0,0 +1,39 @@
+trait Tagged[T]
+
+object Tagged {
+  type Aux[T, UNUSED] = Tagged[T]
+}
+
+trait Fun[R] {
+  type Out
+}
+
+object Fun extends Fun0 {
+  // In Dotty there is a difference between asking for Tagged.Aux[T, Int]
+  // and asking for Tagged[T]. In the former case the companion of T is
+  // not considered as a valid scope during implicit search. In scalac
+  // both cases are treated equally.
+  implicit def tagged[T](implicit t: Tagged.Aux[T, Int]): Fun[T] { type Out = Int } = ???
+}
+
+trait Fun0 {
+  implicit def default[T]: Fun[T] { type Out = String } = ???
+}
+
+object FunDemo extends App {
+  case class A(x: Int, y: String)
+  object A {
+    implicit val tag: Tagged[A] = ???
+  }
+
+  // Precise version of implicitly that keeps type members
+  def the[T <: AnyRef](implicit ev: T): ev.type = ev
+
+  val adhl = the[Fun[A]]
+
+  // Compiles in scalac: the tagged case wins the implicit search using A.tag
+  // Does not compile in Dotty: because of Tagged.Aux[T, _] the companion
+  //                            object of T is not explored during the search,
+  //                            it fallbacks to default (type Out = String)
+  identity[Fun[A] { type Out = Int }](adhl)
+}