Finally figured out what was going on with a ce...

Finally figured out what was going on with a certain class of
exhaustiveness checking bugs. Hey moors, you can put away your pins,
puppets, and magic sauces. Closes #3098, no review.

2 files changed, 26 insertions, 0 deletions

diff --git a/test/files/pos/bug4020.flags b/test/files/pos/bug4020.flags
new file mode 100644
index 0000000000..85d8eb2ba2
--- /dev/null
+++ b/test/files/pos/bug4020.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
diff --git a/test/files/pos/bug4020.scala b/test/files/pos/bug4020.scala
new file mode 100644
index 0000000000..f976460191
--- /dev/null
+++ b/test/files/pos/bug4020.scala
@@ -0,0 +1,25 @@
+class A {
+    sealed trait Foo
+}
+
+object a1 extends A {
+    case class Foo1(i: Int) extends Foo
+}
+
+object a2 extends A {
+    case class Foo2(i: Int) extends Foo
+}
+
+class B {
+    def mthd(foo: a2.Foo) = {
+        foo match {
+            case a2.Foo2(i) => i
+
+            // Note: This case is impossible.  In fact, scalac
+            // will (correctly) report an error if it is uncommented,
+            // but a warning if it is commented.
+
+            // case a1.Foo1(i) => i
+        }
+    }
+}
\ No newline at end of file