Promote unchecked warnings into being emitted by default.

To make that viable, suppression of unchecked warnings is now available
on a per-type-argument basis. The @unchecked annotation has hereby been
generalized beyond exhaustiveness to mean context-dependent "disable further
compiler checking on this entity." Example of new usage:

  def f(x: Any) = x match {
    case xs: List[String @unchecked] => xs.head   // no warning
    case xs: List[Int]               => xs.head   // unchecked warning
  }

It turns out -unchecked has been put to other noisy uses such as
the pattern matcher complaining about its budget like a careworn spouse.
This actually simplified the path forward: I left -unchecked in place
for that and general compatibility, so those warnings can be enabled
as before with -unchecked. The erasure warnings I turned into regular
warnings, subject to suppression by @unchecked.

Review by @odersky.

1 files changed, 14 insertions, 4 deletions

diff --git a/test/files/neg/t3692-new.check b/test/files/neg/t3692-new.check
index 349e4fe48a..5aa991c105 100644
--- a/test/files/neg/t3692-new.check
+++ b/test/files/neg/t3692-new.check
@@ -1,4 +1,14 @@
-t3692-new.scala:16: error: unreachable code

-      case m2: Map[T, Int] => new java.util.HashMap[T, Integer]

-                              ^

-one error found

+t3692-new.scala:14: warning: non-variable type argument Int in type pattern Map[Int,Int] is unchecked since it is eliminated by erasure
+      case m0: Map[Int, Int] => new java.util.HashMap[Integer, Integer]
+               ^
+t3692-new.scala:15: warning: non-variable type argument Int in type pattern Map[Int,V] is unchecked since it is eliminated by erasure
+      case m1: Map[Int, V] => new java.util.HashMap[Integer, V]
+               ^
+t3692-new.scala:16: warning: non-variable type argument Int in type pattern Map[T,Int] is unchecked since it is eliminated by erasure
+      case m2: Map[T, Int] => new java.util.HashMap[T, Integer]
+               ^
+t3692-new.scala:16: error: unreachable code
+      case m2: Map[T, Int] => new java.util.HashMap[T, Integer]
+                              ^
+three warnings found
+one error found