Add missing tests for not-quite-working flow steps

Author: atorralba

swift/ql/test/library-tests/dataflow/dataflow/FlowConfig.qll

@@ -4,6 +4,7 @@
 
 import swift
 import codeql.swift.dataflow.DataFlow
+import codeql.swift.dataflow.ExternalFlow
 
 class TestConfiguration extends DataFlow::Configuration {
   TestConfiguration() { this = "TestConfiguration" }
@@ -14,10 +15,17 @@ class TestConfiguration extends DataFlow::Configuration {
 
   override predicate isSink(DataFlow::Node sink) {
     exists(CallExpr sinkCall |
-      sinkCall.getStaticTarget().getName() = "sink(arg:)" and
+      sinkCall.getStaticTarget().getName() = ["sink(arg:)", "sink(opt:)"] and
       sinkCall.getAnArgument().getExpr() = sink.asExpr()
     )
   }
 
   override int explorationLimit() { result = 100 }
 }
+
+private class TestSummaries extends SummaryModelCsv {
+  override predicate row(string row) {
+    // dumb model to allow testing flow through optional chaining (`x?.signum()`)
+    row = ";Int;true;signum();;;Argument[-1];ReturnValue;value"
+  }
+}

swift/ql/test/library-tests/dataflow/dataflow/test.swift

@@ -251,4 +251,21 @@ func test_computed_property() {
 func test_property_wrapper() {
     @DidSetSource var x = 42
     sink(arg: x) // $ MISSING: flow=243
-}
+}
+
+func sink(opt: Int?) {}
+
+func optionalSource() -> Int? {
+    return source()
+}
+
+func test_optionals() {
+    let x = optionalSource()
+    sink(arg: x!) // $ flow=259
+    sink(arg: source().signum()) // $ MISSING: flow=259
+    sink(opt: x?.signum()) // $ MISSING: flow=259
+    sink(arg: x ?? 0) // $ MISSING: flow=259
+    if let y = x {
+        sink(arg: y) // $ MISSING: flow=259
+    }
+}