Disallow subtypes of Function1 acting as implicit conversions

The new test `falseView.scala` shows the problem. We might create
an implicit value of some type that happens to be a subtype of Function1.
We might now expect that this gives us an implicit conversion, this
is most often unintended and surprising.

See the comment in Implicits#discardForView for a discussion why
we picked the particular scheme implemented here.

Author: odersky

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -300,6 +300,8 @@ class Definitions {
   lazy val ScalaPredefModuleRef = ctx.requiredModuleRef("scala.Predef")
   def ScalaPredefModule(implicit ctx: Context) = ScalaPredefModuleRef.symbol
 
+    lazy val Predef_ConformsR = ScalaPredefModule.requiredClass("$less$colon$less").typeRef
+    def Predef_Conforms(implicit ctx: Context) = Predef_ConformsR.symbol
     lazy val Predef_conformsR = ScalaPredefModule.requiredMethodRef("$conforms")
     def Predef_conforms(implicit ctx: Context) = Predef_conformsR.symbol
     lazy val Predef_classOfR = ScalaPredefModule.requiredMethodRef("classOf")

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

@@ -82,11 +82,33 @@ object Implicits {
           case tpw: TermRef =>
             false // can't discard overloaded refs
           case tpw =>
-            //if (ctx.typer.isApplicable(tp, argType :: Nil, resultType))
-            //  println(i"??? $tp is applicable to $this / typeSymbol = ${tpw.typeSymbol}")
-            !tpw.derivesFrom(defn.FunctionClass(1)) ||
-            ref.symbol == defn.Predef_conforms //
-              // as an implicit conversion, Predef.$conforms is a no-op, so exclude it
+            // Only direct instances of Function1 and direct or indirect instances of <:< are eligible as views.
+            // However, Predef.$conforms is not eligible, because it is a no-op.
+            //
+            // In principle, it would be cleanest if only implicit methods qualified
+            // as implicit conversions. The reasons for deviating are as follows:
+            // Keeping Function1: It's still used quite often (for instance, view
+            // bounds map to implicits of function types) and there is no feasible workaround.
+            // One tempting workaround would be to add a global def
+            //
+            //    implicit def convertIfFuntion1[A, B](x: A)(implicit ev: A => B): B = ev(a)
+            //
+            // But that would throw out the baby with the bathwater. Now, every subtype of
+            // function gives again rise to an implicit conversion. So it's better to just accept
+            // function types in their dual roles.
+            //
+            // The reason for the treatment of <:< and conforms is similar. We could
+            // avoid the clause by having a standard conversion like this in Predef:
+            //
+            //    implicit def convertIfConforms[A, B](x: A)(implicit ev: A <:< B): B = ev(a)
+            //
+            // But that would slow down implicit search a lot, because this conversion is
+            // eligible for all pairs of types, and therefore is tried a lot. So we
+            // emulate the existence of a such a conversion directly in the search.
+            // The reason for leaving out `Predef_conforms` is that we know it adds
+            // nothing since it only relates subtype with supertype.
+            !tpw.isRef(defn.FunctionClass(1)) &&
+            (!tpw.derivesFrom(defn.Predef_Conforms) || ref.symbol == defn.Predef_conforms)
         }
 
         def discardForValueType(tpw: Type): Boolean = tpw match {

tests/neg/falseView.scala

@@ -0,0 +1,7 @@
+object Test {
+
+  private implicit val xs: Map[String, Int] = ???
+
+  val x: Int = "abc" // error
+
+}

tests/pos/t2421_delitedsl.scala

@@ -1,6 +1,9 @@
 trait DeliteDSL {
   abstract class <~<[-From, +To] extends (From => To)
+
   implicit def trivial[A]: A <~< A = new (A <~< A) {def apply(x: A) = x}
+  implicit def convert_<-<[A, B](x: A)(implicit ev: A <~< B): B = ev(x)
+
 
   trait Forcible[T]
   object Forcible {

tests/run/t8280.check

@@ -1,7 +1,6 @@
-Int
-Int
 Long
 Int
 Int
 Int
 Int
+Int

tests/run/t8280.scala

@@ -37,7 +37,8 @@ object Moop1 {
     implicit object f1 extends (Int => String) { def apply(x: Int): String = "Int" }
     implicit val f2: Long => String = _ => "Long"
 
-    println(5: String)
+    //println(5: String)
+      // This picked f1 before, but is now disallowed since subtypes of functions are no longer implicit conversions
   }
 }