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, 3 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/settings/Warnings.scala b/src/compiler/scala/tools/nsc/settings/Warnings.scala
index 16f8685a87..bfa1714894 100644
--- a/src/compiler/scala/tools/nsc/settings/Warnings.scala
+++ b/src/compiler/scala/tools/nsc/settings/Warnings.scala
@@ -29,7 +29,8 @@ trait Warnings {
     warnInaccessible,
     warnNullaryOverride,
     warnNullaryUnit,
-    warnAdaptedArgs
+    warnAdaptedArgs,
+    warnInferAny
   )
 
   // Warning groups.
@@ -52,6 +53,7 @@ trait Warnings {
   val warnInaccessible     = BooleanSetting   ("-Ywarn-inaccessible", "Warn about inaccessible types in method signatures.")
   val warnNullaryOverride  = BooleanSetting   ("-Ywarn-nullary-override",
     "Warn when non-nullary overrides nullary, e.g. `def foo()` over `def foo`.")
+  val warnInferAny         = BooleanSetting   ("-Ywarn-infer-any", "Warn when a type argument is inferred to be `Any`.")
 
   // Backward compatibility.
   def Xwarnfatal    = fatalWarnings