SI-6675 -Xlint arity enforcement for extractors

Extractor Patterns changed in 2.10.0 to implement
the letter of the spec, which allows a single binding
to capture an entire TupleN. But this can hide arity
mismatches, especially if the case body uses the
bound value as an `Any`.

This change warns when this happens under -Xlint.

3 files changed, 18 insertions, 0 deletions

diff --git a/test/files/neg/t6675.check b/test/files/neg/t6675.check
new file mode 100644
index 0000000000..7b271de213
--- /dev/null
+++ b/test/files/neg/t6675.check
@@ -0,0 +1,4 @@
+t6675.scala:10: error: extractor pattern binds a single value to a Product3 of type (Int, Int, Int)
+  "" match { case X(b) => b } // should warn under -Xlint. Not an error because of SI-6111
+                  ^
+one error found
diff --git a/test/files/neg/t6675.flags b/test/files/neg/t6675.flags
new file mode 100644
index 0000000000..e93641e931
--- /dev/null
+++ b/test/files/neg/t6675.flags
@@ -0,0 +1 @@
+-Xlint -Xfatal-warnings
\ No newline at end of file
diff --git a/test/files/neg/t6675.scala b/test/files/neg/t6675.scala
new file mode 100644
index 0000000000..4d500b77ba
--- /dev/null
+++ b/test/files/neg/t6675.scala
@@ -0,0 +1,13 @@
+object X {
+  def unapply(s: String): Option[(Int,Int,Int)] = Some((1,2,3))
+}
+
+object Y {
+  def unapplySeq(s: String): Option[Seq[(Int,Int,Int)]] = Some(Seq((1,2,3)))
+}
+
+object Test {
+  "" match { case X(b) => b } // should warn under -Xlint. Not an error because of SI-6111
+
+  "" match { case Y(b) => b } // no warning
+}