Warn when Any or AnyVal is inferred.

For the very small price of annotating types as Any/AnyVal in those
cases where we wish to use them, we can obtain useful warnings.
I made trunk clean against this warning and found several bugs
or at least suboptimalities in the process.

I put the warning behind -Xlint for the moment, but I think this
belongs on by default, even for this alone:

  scala> List(1, 2, 3) contains "a"
  <console>:8: warning: a type was inferred to be `Any`; this may indicate a programming error.
                List(1, 2, 3) contains "a"
                                       ^
  res0: Boolean = false

Or this punishment meted out by SI-4042:

  scala> 1l to 5l contains 5
  <console>:8: warning: a type was inferred to be `AnyVal`; this may indicate a programming error.
                1l to 5l contains 5
                                  ^
  res0: Boolean = false

A different situation where this arises, which I have seen variations
of many times:

  scala> class A[T](default: T) {
    def get(x: => Option[T]) = x getOrElse Some(default)
  }
  <console>:7: warning: a type was inferred to be `Any`; this may indicate a programming error.
         class A[T](default: T) { def get(x: => Option[T]) = x getOrElse Some(default) }
                                                                             ^
  // Oops, this was what I meant
  scala> class A[T](default: T) {
    def get(x: => Option[T]) = x getOrElse default
  }
  defined class A

Harder to avoid spurious warnings when "Object" is inferred.

1 files changed, 19 insertions, 0 deletions

diff --git a/test/files/neg/warn-inferred-any.scala b/test/files/neg/warn-inferred-any.scala
new file mode 100644
index 0000000000..b853e6e5a8
--- /dev/null
+++ b/test/files/neg/warn-inferred-any.scala
@@ -0,0 +1,19 @@
+trait Foo[-A <: AnyRef, +B <: AnyRef] {
+  def run[U](x: A)(action: B => U): Boolean = ???
+
+  { run(_: A)(_: B => String) }
+}
+
+trait Xs[+A] {
+  { List(1, 2, 3) contains "a" }  // only this warns
+  { List(1, 2, 3) contains 1 }
+  { identity(List(1, 2, 3) contains 1) }
+  { List("a") foreach println }
+}
+
+trait Ys[+A] {
+  { 1 to 5 contains 5l }
+  { 1l to 5l contains 5 }
+  { 1l to 5l contains 5d }
+  { 1l to 5l contains 5l }
+}