SI-6968 Simple Tuple patterns aren't irrefutable

Reverts part of c82ecab. The parser can't assume that
a pattern `(a, b)` will match, as results of
`.isInstanceOf[Tuple2]` can't be statically known until
after the typer.

The reopens SI-1336, SI-5589 and SI-4574, in exchange for fixing
this regression SI-6968. Keeping all of those fixed will require
a better definition of irrefutability, and some acrobatics to
ensure safe passage to the ambiguous trees through typechecking.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/pending/run/t4574.scala b/test/pending/run/t4574.scala
new file mode 100644
index 0000000000..1dde496aca
--- /dev/null
+++ b/test/pending/run/t4574.scala
@@ -0,0 +1,13 @@
+object Test {
+  val xs: List[(Int, Int)] = List((2, 2), null)
+  
+  def expectMatchError[T](msg: String)(body: => T) {
+    try { body ; assert(false, "Should not succeed.") }
+    catch { case _: MatchError => println(msg) }
+  }
+
+  def main(args: Array[String]): Unit = {
+    expectMatchError("I hereby refute null!")( for ((x, y) <- xs) yield x )
+    expectMatchError("I denounce null as unListLike!")( (null: Any) match { case List(_*) => true } )
+  }
+}