Improve unchecked warnings a lot.

Don't warn on "uncheckable" type patterns if they can be
statically guaranteed, regardless of their runtime checkability.
This covers patterns like Seq[Any] and lots more.

Review by @adriaanm.

4 files changed, 100 insertions, 6 deletions

diff --git a/test/files/neg/unchecked.check b/test/files/neg/unchecked.check
new file mode 100644
index 0000000000..34a11db1a0
--- /dev/null
+++ b/test/files/neg/unchecked.check
@@ -0,0 +1,19 @@
+unchecked.scala:18: error: non-variable type argument String in type pattern Iterable[String] is unchecked since it is eliminated by erasure
+    case xs: Iterable[String] => xs.head // unchecked
+             ^
+unchecked.scala:22: error: non-variable type argument Any in type pattern Set[Any] is unchecked since it is eliminated by erasure
+    case xs: Set[Any]    => xs.head // unchecked
+             ^
+unchecked.scala:26: error: non-variable type argument Any in type pattern Map[Any,Any] is unchecked since it is eliminated by erasure
+    case xs: Map[Any, Any] => xs.head // unchecked
+             ^
+unchecked.scala:35: error: non-variable type argument List[Nothing] in type pattern Test.Contra[List[Nothing]] is unchecked since it is eliminated by erasure
+    case xs: Contra[List[Nothing]] => xs.head // unchecked
+             ^
+unchecked.scala:50: error: non-variable type argument String in type pattern Test.Exp[String] is unchecked since it is eliminated by erasure
+    case ArrayApply(x: Exp[Array[T]], _, j: Exp[String]) => x // unchecked
+                                            ^
+unchecked.scala:55: error: non-variable type argument Array[T] in type pattern Test.Exp[Array[T]] is unchecked since it is eliminated by erasure
+    case ArrayApply(x: Exp[Array[T]], _, _) => x // unchecked
+                       ^
+6 errors found
diff --git a/test/files/neg/unchecked.flags b/test/files/neg/unchecked.flags
new file mode 100644
index 0000000000..464cc20ea6
--- /dev/null
+++ b/test/files/neg/unchecked.flags
@@ -0,0 +1 @@
+-Xfatal-warnings -unchecked
\ No newline at end of file
diff --git a/test/files/neg/unchecked.scala b/test/files/neg/unchecked.scala
new file mode 100644
index 0000000000..b50cdf9d7a
--- /dev/null
+++ b/test/files/neg/unchecked.scala
@@ -0,0 +1,74 @@
+import language.existentials
+
+object Test {
+  class Def[T]
+  class Exp[T]
+  class Contra[-T] { def head[T1 <: T] : T1 = ??? }
+  class Cov[+T] { }
+
+  case class ArrayApply[T](x: Exp[Array[T]], i: Exp[Int], j: Exp[_]) extends Def[T]
+
+  val IntArrayApply = ArrayApply[Int](new Exp[Array[Int]], new Exp[Int], new Exp[Int])
+
+  def f(x: Any) = x match {
+    case xs: Iterable[Any]    => xs.head // okay
+    case _                    => 0
+  }
+  def f2(x: Any) = x match {
+    case xs: Iterable[String] => xs.head // unchecked
+    case _                    => 0
+  }
+  def f3(x: Any) = x match {
+    case xs: Set[Any]    => xs.head // unchecked
+    case _                    => 0
+  }
+  def f4(x: Any) = x match {
+    case xs: Map[Any, Any] => xs.head // unchecked
+    case _                 => 0
+  }
+
+  def cf1(x: Any) = x match {
+    case xs: Contra[Nothing] => xs.head // okay
+    case _                   => 0
+  }
+  def cf2(x: Any) = x match {
+    case xs: Contra[List[Nothing]] => xs.head // unchecked
+    case _                         => 0
+  }
+
+  def co1(x: List[Cov[List[Int]]]) = x match {
+    case _: Seq[Cov[Seq[Any]]] => true  // okay
+    case _                     => false
+  }
+
+  def g[T](x: Def[T]) = x match {
+    case ArrayApply(x: Exp[Array[T]], i: Exp[Int], _) => x // okay
+    case _                                            => 0
+  }
+
+  def g2[T](x: Def[T]) = x match {
+    case ArrayApply(x: Exp[Array[T]], _, j: Exp[String]) => x // unchecked
+    case _                                               => 0
+  }
+
+  def g3[T](x: Any) = x match {
+    case ArrayApply(x: Exp[Array[T]], _, _) => x // unchecked
+    case _                                  => 0
+  }
+
+  def g4 = IntArrayApply match {
+    case ArrayApply(x: Exp[Array[Int]], _, _) => x // okay
+    case _                                    => ()
+  }
+  def g5[T](x: ArrayApply[Int]) = x match {
+    case ArrayApply(x: Exp[Array[Int]], _, _) => x // okay
+    case _                                    => 0
+  }
+
+  // Nope
+  //
+  // def g5 = IntArrayApply match {
+  //   case ArrayApply(x: Exp[Array[String]], _, _) => x // nope
+  //   case _                                       => ()
+  // }
+}
diff --git a/test/files/neg/unchecked2.check b/test/files/neg/unchecked2.check
index 2c0be9ce00..e37865928e 100644
--- a/test/files/neg/unchecked2.check
+++ b/test/files/neg/unchecked2.check
@@ -1,19 +1,19 @@
-unchecked2.scala:2: error: non variable type-argument Int in type Option[Int] is unchecked since it is eliminated by erasure
+unchecked2.scala:2: error: non-variable type argument Int in type Option[Int] is unchecked since it is eliminated by erasure
   Some(123).isInstanceOf[Option[Int]]
                         ^
-unchecked2.scala:3: error: non variable type-argument String in type Option[String] is unchecked since it is eliminated by erasure
+unchecked2.scala:3: error: non-variable type argument String in type Option[String] is unchecked since it is eliminated by erasure
   Some(123).isInstanceOf[Option[String]]
                         ^
-unchecked2.scala:4: error: non variable type-argument List[String] in type Option[List[String]] is unchecked since it is eliminated by erasure
+unchecked2.scala:4: error: non-variable type argument List[String] in type Option[List[String]] is unchecked since it is eliminated by erasure
   Some(123).isInstanceOf[Option[List[String]]]
                         ^
-unchecked2.scala:5: error: non variable type-argument List[Int => String] in type Option[List[Int => String]] is unchecked since it is eliminated by erasure
+unchecked2.scala:5: error: non-variable type argument List[Int => String] in type Option[List[Int => String]] is unchecked since it is eliminated by erasure
   Some(123).isInstanceOf[Option[List[Int => String]]]
                         ^
-unchecked2.scala:6: error: non variable type-argument (String, Double) in type Option[(String, Double)] is unchecked since it is eliminated by erasure
+unchecked2.scala:6: error: non-variable type argument (String, Double) in type Option[(String, Double)] is unchecked since it is eliminated by erasure
   Some(123).isInstanceOf[Option[(String, Double)]]
                         ^
-unchecked2.scala:7: error: non variable type-argument String => Double in type Option[String => Double] is unchecked since it is eliminated by erasure
+unchecked2.scala:7: error: non-variable type argument String => Double in type Option[String => Double] is unchecked since it is eliminated by erasure
   Some(123).isInstanceOf[Option[String => Double]]
                         ^
 6 errors found