Add extension method dataflow tests

java/ql/test/kotlin/library-tests/dataflow/extensionMethod/test.expected

@@ -0,0 +1,2 @@
+| test.kt:20:29:20:31 | new C(...) | test.kt:23:22:23:28 | self1(...) |
+| test.kt:20:29:20:31 | new C(...) | test.kt:29:18:29:29 | fn1(...) |

java/ql/test/kotlin/library-tests/dataflow/extensionMethod/test.kt

@@ -0,0 +1,40 @@
+class C {
+    fun self1() = this
+    fun fn1(o: C) = o
+
+    fun Int.fn3(o: C) = o
+    fun Int.fn4() = this@C
+
+    fun call1(o: C) = 1.fn3(o)
+    fun call2() = 1.fn4()
+}
+
+fun C.self2() = this
+fun C.fn2(o: C) = o
+
+class Test {
+    fun <T> taint(t: T) = t
+    fun sink(a: Any) {}
+
+    fun test(s1: String) {
+        val tainted = taint(C())
+
+        sink(C().self1())
+        sink(tainted.self1())
+
+        sink(C().self2())
+        sink(tainted.self2())
+
+        sink(C().fn1(C()))
+        sink(C().fn1(tainted))
+
+        sink(C().fn2(C()))
+        sink(C().fn2(tainted))
+
+        sink(C().call1(C()))
+        sink(C().call1(tainted))
+
+        sink(C().call2())
+        sink(tainted.call2())
+    }
+}

java/ql/test/kotlin/library-tests/dataflow/extensionMethod/test.ql

@@ -0,0 +1,19 @@
+import java
+import semmle.code.java.dataflow.TaintTracking
+import semmle.code.java.dataflow.ExternalFlow
+
+class Conf extends TaintTracking::Configuration {
+  Conf() { this = "qltest:extension-method" }
+
+  override predicate isSource(DataFlow::Node n) {
+    n.asExpr().(Argument).getCall().getCallee().hasName("taint")
+  }
+
+  override predicate isSink(DataFlow::Node n) {
+    n.asExpr().(Argument).getCall().getCallee().hasName("sink")
+  }
+}
+
+from DataFlow::Node src, DataFlow::Node sink, Conf conf
+where conf.hasFlow(src, sink)
+select src, sink