Fix for SI-6537, inaccurate unchecked warning.

I found a more direct expression of the unchecked logic,
which should be much easier for others to verify.  But the
bug being fixed here is that the unchecked checking happens
too early, and the sealed children of a symbol are not yet
visible if it is being simultaneously compiled.

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
+}