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.

5 files changed, 55 insertions, 15 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
diff --git a/test/files/neg/t3692-old.check b/test/files/neg/t3692-old.check
index 92d71f7e4e..950f82951c 100644
--- a/test/files/neg/t3692-old.check
+++ b/test/files/neg/t3692-old.check
@@ -1,11 +1,20 @@
-t3692-old.scala:11: warning: type Manifest in object Predef is deprecated: Use scala.reflect.ClassTag (to capture erasures) or scala.reflect.runtime.universe.TypeTag (to capture types) or both instead

-  private final def toJavaMap[T, V](map: Map[T, V])(implicit m1: Manifest[T], m2: Manifest[V]): java.util.Map[_, _] = {

-                                                                 ^

-t3692-old.scala:11: warning: type Manifest in object Predef is deprecated: Use scala.reflect.ClassTag (to capture erasures) or scala.reflect.runtime.universe.TypeTag (to capture types) or both instead

-  private final def toJavaMap[T, V](map: Map[T, V])(implicit m1: Manifest[T], m2: Manifest[V]): java.util.Map[_, _] = {

-                                                                                  ^

-t3692-old.scala:15: error: unreachable code

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

-                              ^

-two warnings found

-one error found

+t3692-old.scala:13: 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-old.scala:14: 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-old.scala:15: 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-old.scala:11: warning: type Manifest in object Predef is deprecated: Use scala.reflect.ClassTag (to capture erasures) or scala.reflect.runtime.universe.TypeTag (to capture types) or both instead
+  private final def toJavaMap[T, V](map: Map[T, V])(implicit m1: Manifest[T], m2: Manifest[V]): java.util.Map[_, _] = {
+                                                                 ^
+t3692-old.scala:11: warning: type Manifest in object Predef is deprecated: Use scala.reflect.ClassTag (to capture erasures) or scala.reflect.runtime.universe.TypeTag (to capture types) or both instead
+  private final def toJavaMap[T, V](map: Map[T, V])(implicit m1: Manifest[T], m2: Manifest[V]): java.util.Map[_, _] = {
+                                                                                  ^
+t3692-old.scala:15: error: unreachable code
+      case m2: Map[T, Int] => new java.util.HashMap[T, Integer]
+                              ^
+5 warnings found
+one error found
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
diff --git a/test/files/neg/unchecked-suppress.flags b/test/files/neg/unchecked-suppress.flags
new file mode 100644
index 0000000000..e8fb65d50c
--- /dev/null
+++ b/test/files/neg/unchecked-suppress.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
\ No newline at end of file
diff --git a/test/files/neg/unchecked-suppress.scala b/test/files/neg/unchecked-suppress.scala
new file mode 100644
index 0000000000..7bd61a2a4d
--- /dev/null
+++ b/test/files/neg/unchecked-suppress.scala
@@ -0,0 +1,10 @@
+class A {
+  def f(x: Any) = x match {
+    case xs: List[String @unchecked]               => xs.head   // okay
+    case xs: Set[Int]                              => xs.head   // unchecked
+    case xs: Map[String @unchecked, String]        => xs.head   // one unchecked, one okay
+    case f: ((Int @unchecked) => (Int @unchecked)) => f(5)      // okay
+    case f: ((Int, Int) => Int)                    =>           // unchecked
+    case _                                         => ""
+  }
+}