SI-6899, prohibit dangerous, useless implicit conversions.

Increase eligibility requirements for implicit conversions,
such that T => U is ineligible if

  T <: Null    <or>   AnyRef <: U

This has the salutary effect of allowing us to ditch 16
ridiculous implicits from Predef, since they existed solely
to work around the absence of this restriction.

There was one tiny impact on actual source code (one line
in one file) shown here, necessitated because the literal null
is not eligible to be implicitly converted to A via <:<.

  def f[A](implicit ev: Null <:< A): A = null     // before
  def f[A](implicit ev: Null <:< A): A = ev(null) // after

As impositions go it's on the tame side.

1 files changed, 1 insertions, 7 deletions

diff --git a/test/files/neg/no-implicit-to-anyref.check b/test/files/neg/no-implicit-to-anyref.check
index d94b57a30a..fe417ad8b0 100644
--- a/test/files/neg/no-implicit-to-anyref.check
+++ b/test/files/neg/no-implicit-to-anyref.check
@@ -1,10 +1,4 @@
-no-implicit-to-anyref.scala:11: error: type mismatch;
- found   : Int(1)
- required: AnyRef
-Note: an implicit exists from scala.Int => java.lang.Integer, but
-methods inherited from Object are rendered ambiguous.  This is to avoid
-a blanket implicit which would convert any scala.Int to any AnyRef.
-You may wish to use a type ascription: `x: java.lang.Integer`.
+no-implicit-to-anyref.scala:11: error: the result type of an implicit conversion must be more specific than AnyRef
     1: AnyRef
     ^
 no-implicit-to-anyref.scala:17: error: type mismatch;