Add tests to track nullness by instanceof checks.

java/ql/test/query-tests/Nullness/B.java

@@ -324,4 +324,24 @@ public class B {
     if (x != null) y.hashCode(); // OK
     if (y != null) x.hashCode(); // OK
   }
+
+  public void corrConds3(Object y) {
+    Object x = null;
+    if(y instanceof String) {
+      x = new Object();
+    }
+    if(y instanceof String) {
+      x.hashCode(); // OK
+    }
+  }
+
+  public void corrConds4(Object y) {
+    Object x = null;
+    if(!(y instanceof String)) {
+      x = new Object();
+    }
+    if(!(y instanceof String)) {
+      x.hashCode(); // OK
+    }
+  }
 }

java/ql/test/query-tests/Nullness/NullMaybe.expected

@@ -17,6 +17,8 @@
 | B.java:190:7:190:7 | o | Variable $@ may be null here because of $@ assignment. | B.java:178:5:178:20 | Object o | o | B.java:186:5:186:12 | ...=... | this |
 | B.java:279:7:279:7 | a | Variable $@ may be null here because of $@ assignment. | B.java:276:5:276:19 | int[] a | a | B.java:276:11:276:18 | a | this |
 | B.java:292:7:292:7 | b | Variable $@ may be null here because of $@ assignment. | B.java:287:5:287:44 | int[] b | b | B.java:287:11:287:43 | b | this |
+| B.java:334:7:334:7 | x | Variable $@ may be null here because of $@ assignment. | B.java:329:5:329:20 | Object x | x | B.java:329:12:329:19 | x | this |
+| B.java:344:7:344:7 | x | Variable $@ may be null here because of $@ assignment. | B.java:339:5:339:20 | Object x | x | B.java:339:12:339:19 | x | this |
 | C.java:9:44:9:45 | a2 | Variable $@ may be null here as suggested by $@ null guard. | C.java:6:5:6:23 | long[][] a2 | a2 | C.java:7:34:7:54 | ... != ... | this |
 | C.java:9:44:9:45 | a2 | Variable $@ may be null here because of $@ assignment. | C.java:6:5:6:23 | long[][] a2 | a2 | C.java:6:14:6:22 | a2 | this |
 | C.java:10:17:10:18 | a3 | Variable $@ may be null here as suggested by $@ null guard. | C.java:8:5:8:21 | long[] a3 | a3 | C.java:9:38:9:58 | ... != ... | this |