SI-6675 Avoid spurious warning about pattern bind arity.

In 692372ce, we added a warning (under -Xlint) when binding
a `TupleN` in to a single pattern binder, which wasn't allowed
before 2.10.0, and more often than not represents a bug.

However, that warning overstretched, and warned even when
using a Tuple Pattern to bind to the elements of such a value.

This commit checks for this case, and avoids the spurious warnings.

A new test case is added for this case to go with the existing
test for SI-6675:

    $ ./tools/partest-ack 6675
    % tests-with-matching-paths      ...  3
    % tests-with-matching-code       ...  2
    # 3 tests to run.
    test/partest --show-diff --show-log                  \
      test/files/neg/t6675-old-patmat.scala              \
      test/files/neg/t6675.scala                         \
      test/files/pos/t6675.scala                         \
      ""

    Testing individual files
    testing: [...]/files/pos/t6675.scala                                  [  OK  ]

    Testing individual files
    testing: [...]/files/neg/t6675-old-patmat.scala                       [  OK  ]
    testing: [...]/files/neg/t6675.scala                                  [  OK  ]
    All of 3 tests were successful (elapsed time: 00:00:03)

2 files changed, 21 insertions, 0 deletions

diff --git a/test/files/pos/t6675.flags b/test/files/pos/t6675.flags
new file mode 100644
index 0000000000..e8fb65d50c
--- /dev/null
+++ b/test/files/pos/t6675.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
\ No newline at end of file
diff --git a/test/files/pos/t6675.scala b/test/files/pos/t6675.scala
new file mode 100644
index 0000000000..f3bebea5be
--- /dev/null
+++ b/test/files/pos/t6675.scala
@@ -0,0 +1,20 @@
+object LeftOrRight {
+  def unapply[A](value: Either[A, A]): Option[A] = value match {
+    case scala.Left(x) => Some(x)
+    case scala.Right(x) => Some(x)
+  }
+}
+
+object Test {
+  (Left((0, 0)): Either[(Int, Int), (Int, Int)]) match {
+    case LeftOrRight(pair @ (a, b)) => a // false -Xlint warning: "extractor pattern binds a single value to a Product2 of type (Int, Int)"
+  }
+
+  (Left((0, 0)): Either[(Int, Int), (Int, Int)]) match {
+    case LeftOrRight((a, b)) => a // false -Xlint warning: "extractor pattern binds a single value to a Product2 of type (Int, Int)"
+  }
+
+  (Left((0, 0)): Either[(Int, Int), (Int, Int)]) match {
+    case LeftOrRight(a, b) => a // false -Xlint warning: "extractor pattern binds a single value to a Product2 of type (Int, Int)"
+  }
+}