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, 10 insertions, 0 deletions

diff --git a/test/files/neg/unchecked-suppress.check b/test/files/neg/unchecked-suppress.check
new file mode 100644
index 0000000000..2e23d21386
--- /dev/null
+++ b/test/files/neg/unchecked-suppress.check
@@ -0,0 +1,10 @@
+unchecked-suppress.scala:4: error: non-variable type argument Int in type pattern Set[Int] is unchecked since it is eliminated by erasure
+    case xs: Set[Int]                              => xs.head   // unchecked
+             ^
+unchecked-suppress.scala:5: error: non-variable type argument String in type pattern Map[String @unchecked,String] is unchecked since it is eliminated by erasure
+    case xs: Map[String @unchecked, String]        => xs.head   // one unchecked, one okay
+             ^
+unchecked-suppress.scala:7: error: non-variable type argument Int in type pattern (Int, Int) => Int is unchecked since it is eliminated by erasure
+    case f: ((Int, Int) => Int)                    =>           // unchecked
+                        ^
+three errors found