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.

1 files changed, 7 insertions, 0 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
+
+}