4 files changed, 25 insertions, 3 deletions
diff --git a/test/files/neg/unchecked-knowable.check b/test/files/neg/unchecked-knowable.check
index 3a6ef994b5..d279427327 100644
--- a/test/files/neg/unchecked-knowable.check
+++ b/test/files/neg/unchecked-knowable.check
@@ -1,4 +1,7 @@
-unchecked-knowable.scala:17: error: fruitless type test: a value of type Bippy cannot also be a A1
+unchecked-knowable.scala:18: error: fruitless type test: a value of type Bippy cannot also be a A1
   /*   warn */ (new Bippy).isInstanceOf[A1]
                                        ^
-one error found
+unchecked-knowable.scala:19: error: fruitless type test: a value of type Bippy cannot also be a B1
+  /*   warn */ (new Bippy).isInstanceOf[B1]
+                                       ^
+two errors found
diff --git a/test/files/neg/unchecked-knowable.scala b/test/files/neg/unchecked-knowable.scala
index 667b47f504..21624c4fb4 100644
--- a/test/files/neg/unchecked-knowable.scala
+++ b/test/files/neg/unchecked-knowable.scala
@@ -7,6 +7,7 @@ final class A4 extends A2
 /** Unknowable */
 sealed abstract class B1
 sealed abstract class B2 extends B1
+sealed trait B2B extends B1
 final class B3 extends B1
 trait B4 extends B2
 
@@ -15,6 +16,7 @@ trait Dingus
 
 class A {
   /*   warn */ (new Bippy).isInstanceOf[A1]
-  /* nowarn */ (new Bippy).isInstanceOf[B1]
+  /*   warn */ (new Bippy).isInstanceOf[B1]
+  /* nowarn */ (null: Dingus).isInstanceOf[B1]
   /* nowarn */ ((new Bippy): Any).isInstanceOf[A1]
 }
diff --git a/test/files/pos/t6537.flags b/test/files/pos/t6537.flags
new file mode 100644
index 0000000000..85d8eb2ba2
--- /dev/null
+++ b/test/files/pos/t6537.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
diff --git a/test/files/pos/t6537.scala b/test/files/pos/t6537.scala
new file mode 100644
index 0000000000..d0ca3ba435
--- /dev/null
+++ b/test/files/pos/t6537.scala
@@ -0,0 +1,16 @@
+package tester
+
+object PatMatWarning {
+
+  sealed trait X
+  sealed trait Y
+
+  def f(x: X) = x match {
+    case _: Y => false
+    case _    => true
+  }
+
+  class X1 extends X
+  class Y1 extends Y
+  class Z1 extends X with Y
+}