Java: Add test.

java/ql/test/library-tests/dataflow/typepruning/A.java

@@ -0,0 +1,18 @@
+public class A {
+  Object customStep(Object o) { return null; }
+
+  Object source() { return null; }
+
+  void sink(Object o) { }
+
+  Object through1(Object o) {
+    Object o2 = customStep(o);
+    String s = (String)o2;
+    return s;
+  }
+
+  void foo() {
+    Object x = through1(source());
+    sink(x);
+  }
+}

java/ql/test/library-tests/dataflow/typepruning/test.expected

@@ -0,0 +1 @@
+| A.java:15:25:15:32 | source(...) | A.java:16:10:16:10 | x | A.java:14:8:14:10 | foo |

java/ql/test/library-tests/dataflow/typepruning/test.ql

@@ -0,0 +1,27 @@
+import java
+import semmle.code.java.dataflow.DataFlow
+import DataFlow
+
+class Conf extends Configuration {
+  Conf() { this = "test types" }
+
+  override predicate isSource(Node n) { n.asExpr().(MethodAccess).getMethod().hasName("source") }
+
+  override predicate isSink(Node n) {
+    exists(MethodAccess sink |
+      sink.getAnArgument() = n.asExpr() and sink.getMethod().hasName("sink")
+    )
+  }
+
+  override predicate isAdditionalFlowStep(Node n1, Node n2) {
+    exists(MethodAccess ma |
+      ma.getMethod().hasName("customStep") and
+      ma.getAnArgument() = n1.asExpr() and
+      ma = n2.asExpr()
+    )
+  }
+}
+
+from Node src, Node sink, Conf conf
+where conf.hasFlow(src, sink)
+select src, sink, sink.getEnclosingCallable()