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.

5 files changed, 29 insertions, 28 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;
diff --git a/test/files/neg/t4158.check b/test/files/neg/t4158.check
index 3ee2627c5b..af281c52cd 100644
--- a/test/files/neg/t4158.check
+++ b/test/files/neg/t4158.check
@@ -1,19 +1,7 @@
-t4158.scala:3: error: type mismatch;
- found   : Null(null)
- required: Int
-Note that implicit conversions are not applicable because they are ambiguous:
- both method Integer2intNullConflict in class LowPriorityImplicits of type (x: Null)Int
- and method Integer2int in object Predef of type (x: Integer)Int
- are possible conversion functions from Null(null) to Int
+t4158.scala:3: error: an expression of type Null is ineligible for implicit conversion
   var y = null: Int
           ^
-t4158.scala:2: error: type mismatch;
- found   : Null(null)
- required: Int
-Note that implicit conversions are not applicable because they are ambiguous:
- both method Integer2intNullConflict in class LowPriorityImplicits of type (x: Null)Int
- and method Integer2int in object Predef of type (x: Integer)Int
- are possible conversion functions from Null(null) to Int
+t4158.scala:2: error: an expression of type Null is ineligible for implicit conversion
   var x: Int = null
                ^
 two errors found
diff --git a/test/files/neg/t4727.check b/test/files/neg/t4727.check
index 8a4536fec3..a17cdde044 100644
--- a/test/files/neg/t4727.check
+++ b/test/files/neg/t4727.check
@@ -1,10 +1,4 @@
-t4727.scala:5: error: type mismatch;
- found   : Null
- required: Int
-Note that implicit conversions are not applicable because they are ambiguous:
- both method Integer2intNullConflict in class LowPriorityImplicits of type (x: Null)Int
- and method Integer2int in object Predef of type (x: Integer)Int
- are possible conversion functions from Null to Int
+t4727.scala:5: error: an expression of type Null is ineligible for implicit conversion
 Error occurred in an application involving default arguments.
     new C[Int]
     ^
diff --git a/test/files/neg/t6889.check b/test/files/neg/t6889.check
new file mode 100644
index 0000000000..a77e8a010c
--- /dev/null
+++ b/test/files/neg/t6889.check
@@ -0,0 +1,7 @@
+t6889.scala:16: error: the result type of an implicit conversion must be more specific than AnyRef
+  def f(x: Dingo): AnyRef = x   // fail - no conversion to AnyRef
+                            ^
+t6889.scala:17: error: an expression of type Null is ineligible for implicit conversion
+  var x: Int = null             // fail - no conversion from Null
+               ^
+two errors found
diff --git a/test/files/neg/t6889.scala b/test/files/neg/t6889.scala
new file mode 100644
index 0000000000..ef1963669c
--- /dev/null
+++ b/test/files/neg/t6889.scala
@@ -0,0 +1,18 @@
+package bippy {
+  trait Bippy[A] extends Any
+}
+package foo {
+  package object unrelated {
+    implicit def bippyDingo[A](x: bippy.Bippy[A]): AnyRef = Nil
+  }
+  package unrelated {
+    trait Unrelated
+  }
+}
+
+object Test {
+  trait Dingo extends Any with bippy.Bippy[foo.unrelated.Unrelated]
+
+  def f(x: Dingo): AnyRef = x   // fail - no conversion to AnyRef
+  var x: Int = null             // fail - no conversion from Null
+}