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.

4 files changed, 13 insertions, 3 deletions

diff --git a/tests/neg/falseView.scala b/tests/neg/falseView.scala
new file mode 100644
index 000000000..613abe3f1
--- /dev/null
+++ b/tests/neg/falseView.scala
@@ -0,0 +1,7 @@
+object Test {
+
+  private implicit val xs: Map[String, Int] = ???
+
+  val x: Int = "abc" // error
+
+}
diff --git a/tests/pos/t2421_delitedsl.scala b/tests/pos/t2421_delitedsl.scala
index 22f1ecd85..bde3593c9 100644
--- a/tests/pos/t2421_delitedsl.scala
+++ b/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 {
diff --git a/tests/run/t8280.check b/tests/run/t8280.check
index 44c51f5aa..b5885df48 100644
--- a/tests/run/t8280.check
+++ b/tests/run/t8280.check
@@ -1,7 +1,6 @@
-Int
-Int
 Long
 Int
 Int
 Int
 Int
+Int
diff --git a/tests/run/t8280.scala b/tests/run/t8280.scala
index f433dcc32..1d2c56b85 100644
--- a/tests/run/t8280.scala
+++ b/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
   }
 }