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, 22 insertions, 0 deletions

diff --git a/test/pending/pos/t5589.scala b/test/pending/pos/t5589.scala
new file mode 100644
index 0000000000..69cbb20391
--- /dev/null
+++ b/test/pending/pos/t5589.scala
@@ -0,0 +1,22 @@
+class A {
+  // First three compile.
+  def f1(x: Either[Int, String])        = x.right map (y => y)
+  def f2(x: Either[Int, String])        = for (y <- x.right) yield y
+  def f3(x: Either[Int, (String, Int)]) = x.right map { case (y1, y2) => (y1, y2) }
+  // Last one fails.
+  def f4(x: Either[Int, (String, Int)]) = for ((y1, y2) <- x.right) yield ((y1, y2))
+/**
+./a.scala:5: error: constructor cannot be instantiated to expected type;
+ found   : (T1, T2)
+ required: Either[Nothing,(String, Int)]
+  def f4(x: Either[Int, (String, Int)]) = for ((y1, y2) <- x.right) yield ((y1, y2))
+                                               ^
+./a.scala:5: error: not found: value y1
+  def f4(x: Either[Int, (String, Int)]) = for ((y1, y2) <- x.right) yield ((y1, y2))
+                                                                            ^
+./a.scala:5: error: not found: value y2
+  def f4(x: Either[Int, (String, Int)]) = for ((y1, y2) <- x.right) yield ((y1, y2))
+                                                                                ^
+three errors found
+**/
+}