Merge pull request #2739 from dotty-staging/fix-ho-implicits

Fix spurious shadowing for higher-order implicits

Author: odersky

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

@@ -752,10 +752,12 @@ trait Implicits { self: Typer =>
         lazy val shadowing =
           typed(untpd.Ident(ref.name) withPos pos.toSynthetic, funProto)(
             nestedContext.addMode(Mode.ImplicitShadowing).setExploreTyperState)
-        def refMatches(shadowing: Tree): Boolean =
+        def refSameAs(shadowing: Tree): Boolean =
           ref.symbol == closureBody(shadowing).symbol || {
             shadowing match {
-              case Trees.Select(qual, nme.apply) => refMatches(qual)
+              case Trees.Select(qual, nme.apply) => refSameAs(qual)
+              case Trees.Apply(fn, _) => refSameAs(fn)
+              case Trees.TypeApply(fn, _) => refSameAs(fn)
               case _ => false
             }
           }
@@ -782,7 +784,7 @@ trait Implicits { self: Typer =>
         if (ctx.reporter.hasErrors)
           nonMatchingImplicit(ref, ctx.reporter.removeBufferedMessages)
         else if (contextual && !ctx.mode.is(Mode.ImplicitShadowing) &&
-                 !shadowing.tpe.isError && !refMatches(shadowing)) {
+                 !shadowing.tpe.isError && !refSameAs(shadowing)) {
           implicits.println(i"SHADOWING $ref in ${ref.termSymbol.owner} is shadowed by $shadowing in ${shadowing.symbol.owner}")
           shadowedImplicit(ref, methPart(shadowing).tpe)
         }

tests/pos/ho-implicits.scala

@@ -0,0 +1,9 @@
+object Test2 {
+
+  implicit def __1: implicit Int => String = s"implicit: ${implicitly[Int]}"
+  implicit def __2: Int = 42
+
+  def f: implicit String => Int = implicitly[String].length
+
+  f: Int
+}

tests/pos/implicitFuns.scala

@@ -0,0 +1,69 @@
+package cm2
+
+case class Person(name: String)
+case class Paper(title: String, authors: List[Person], body: String)
+
+class Viewers(val persons: Set[Person])
+
+class ConfManagement(papers: List[Paper], realScore: Map[Paper, Int]) extends App {
+
+  private def hasConflict(ps1: Set[Person], ps2: Iterable[Person]) =
+    ps2.exists(ps1 contains _)
+
+  type Viewable[T] = implicit Viewers => T
+
+  def vs: Viewable[Viewers] = implicitly
+
+  def viewers: Viewable[Set[Person]] = vs.persons
+
+  def score: Paper => Viewable[Int] =
+    paper =>
+      if hasConflict(viewers, paper.authors) then -100
+      else realScore(paper)
+
+  def viewRankings: Viewable[List[(String, Int)]] =
+    papers.sortBy(-score(_)).map(p => (p.title, score(p)))
+
+  def delegate[T]: (Viewers => T) => Person => Viewable[T] =
+    query => p => query(new Viewers(viewers + p))
+}
+
+object Test extends App {
+  def bob = Person("Bob")
+  def peter = Person("Peter")
+  def p1 = Paper("Bob's paper", List(bob), "")
+  def p2 = Paper("Peter's paper", List(peter), "")
+
+  implicit def __1: Viewers = new Viewers(Set(bob))
+
+  val cm = new ConfManagement(List(p1, p2), Map(p1 -> 2, p2 -> 3))
+
+  println(cm.viewRankings)
+  println(cm.score(p1))
+  println(Orderings.isLess(Nil)(List(1, 2, 3)))
+}
+
+object Orderings extends App {
+
+  trait Ord[T] { def less: T => T => Boolean }
+
+  implicit def __1: Ord[Int] = new Ord[Int] {
+    def less: Int => Int => Boolean =
+      x => y => x < y
+  }
+
+  implicit def __2[T]: implicit Ord[T] => Ord[List[T]] = new Ord[List[T]] {
+    def less: List[T] => List[T] => Boolean =
+      xs => ys =>
+        if ys.isEmpty then false
+        else if xs.isEmpty then true
+        else if xs.head == ys.head then less(xs.tail)(ys.tail)
+        else isLess(xs.head)(ys.head)
+  }
+
+  def isLess[T]: T => T => implicit Ord[T] => Boolean =
+    x => y => implicitly[Ord[T]].less(x)(y)
+
+  println(isLess(Nil)(List(1, 2, 3)))
+  println(isLess(List(List(1)))(List(List(1))))
+}

tests/run/cochis.scala

@@ -0,0 +1,15 @@
+import Predef.{$conforms => _, _}
+
+trait A {
+
+  implicit def id[A] : A => A  =  x => x // (1)
+  def trans[A] (x : A) (implicit f : A => A) = f(x)
+}
+object Test extends A with App {
+  implicit def succ : Int ⇒ Int = x ⇒ x + 1 // (3)
+  def bad [A] (x : A) : A = trans[A](x) // (4) incoherent de€nition !
+  val v1 = bad [Int] (3) // (5) evaluates to 3
+  val v2 = trans [Int] (3) // (6) substituting bad by trans is rejected
+  assert(v1 == 3)
+  assert(v2 == 4)
+}