Dataflow: Add test highlighting missing subpath.

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

@@ -0,0 +1,29 @@
+import java.util.function.*;
+
+class A {
+  Object source(String label) { return null; }
+
+  void sink(Object o) { }
+
+  <T> T propagateTaint(Object arg) {
+    return (T)arg;
+  }
+
+  void test() {
+    // test type strengthening on outgoing through-flow edge
+    String s = this.<String>propagateTaint(source("A"));
+    sink(s); // $ hasValueFlow=A
+
+    // no strengthening
+    Object o = this.<Object>propagateTaint(source("B"));
+    sink(o); // $ hasValueFlow=B
+
+    // test type strengthening on ingoing through-flow edge
+    String s2 = apply((String arg) -> arg, source("C"));
+    sink(s2); // $ hasValueFlow=C
+  }
+
+  <T1, T2> T2 apply(Function<T1, T2> f, Object x) {
+    return f.apply((T1)x);
+  }
+}