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, 7 insertions, 0 deletions

diff --git a/test/files/run/t6968.scala b/test/files/run/t6968.scala
new file mode 100644
index 0000000000..b5cadfd9e1
--- /dev/null
+++ b/test/files/run/t6968.scala
@@ -0,0 +1,7 @@
+object Test {
+  def main(args: Array[String]) {
+    val mixedList = List(1,(1,2),4,(3,1),(5,4),6)
+    val as = for((a,b) <- mixedList) yield a
+    println(as.mkString(", "))
+  }
+}