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/neg/t5589neg2.scala b/test/pending/neg/t5589neg2.scala
new file mode 100644
index 0000000000..b7c7ab7218
--- /dev/null
+++ b/test/pending/neg/t5589neg2.scala
@@ -0,0 +1,13 @@
+class A {
+  def f1(x: List[((((Int, (Double, (Float, String))), List[String]), List[Int]), List[Float])]) = {
+    for (((((a, (b, (c, d))), es), fs), gs) <- x) yield (d :: es).mkString(", ") // ok
+  }
+
+  def f2(x: List[((((Int, (Double, (Float, String))), List[String]), List[Int]), List[Float])]) = {
+    for (((((a, (b, (c, (d1, d2)))), es), fs), gs) <- x) yield (d :: es).mkString(", ") // not ok
+  }
+
+  def f3(x: List[((((Int, (Double, (Float, String))), List[String]), List[Int]), List[Float])]) = {
+    for (((((a, (b, _)), es), fs), gs) <- x) yield (es ::: fs).mkString(", ") // ok
+  }
+}
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