C#: Add flow test cases for undetected value flow, when making variab…

…le bindinds in pattern matching.

csharp/ql/test/library-tests/dataflow/fields/K.cs

@@ -0,0 +1,54 @@
+using System;
+
+public record class RecordClass2(object Prop) { }
+
+public record class Nested(RecordClass2 Record) { }
+
+public class K
+{
+    private void M1()
+    {
+        var o = Source<object>(1);
+        var r = new RecordClass2(o);
+        if (r is RecordClass2 { Prop: object p })
+        {
+            Sink(p); // $ MISSING: hasValueFlow=1
+        }
+    }
+
+    private void M2()
+    {
+        var o = Source<object>(2);
+        var r = new RecordClass2(o);
+        switch (r)
+        {
+            case RecordClass2 { Prop: object p }:
+                Sink(p); // $ MISSING: hasValueFlow=2
+                break;
+        }
+    }
+
+    private void M3()
+    {
+        var o = Source<object>(3);
+        var s = new Nested(new RecordClass2(o));
+        if (s is Nested { Record: { Prop: object p } })
+        {
+            Sink(p); // $ MISSING: hasValueFlow=3
+        }
+    }
+
+    private void M4()
+    {
+        var o = Source<object>(4);
+        var s = new Nested(new RecordClass2(o));
+        if (s is Nested { Record.Prop: object p })
+        {
+            Sink(p); // $ MISSING: hasValueFlow=4
+        }
+    }
+
+    public static void Sink(object o) { }
+
+    static T Source<T>(object source) => throw null;
+}