C++: Fix the XMLParent Constructor.

Author: geoffw0

cpp/ql/src/semmle/code/cpp/XML.qll

@@ -31,6 +31,12 @@ abstract class XMLLocatable extends @xmllocatable {
  * both of which can contain other elements.
  */
 class XMLParent extends @xmlparent {
+  XMLParent() {
+    // explicitly restrict `this` to be either an `XMLElement` or an `XMLFile`;
+    // the type `@xmlparent` currently also includes non-XML files
+    this instanceof @xmlelement or xmlEncoding(this, _)
+  }
+
   /**
    * Gets a printable representation of this XML parent.
    * (Intended to be overridden in subclasses.)

cpp/ql/test/library-tests/files/Files.expected

@@ -1,4 +1,4 @@
-| c.c | library-tests/files/c.c | CFile, MetricFile | C, XMLParent |  |  |
-| files1.cpp | library-tests/files/files1.cpp | CppFile, MetricFile | C++, XMLParent | swap | t |
-| files1.h | library-tests/files/files1.h | HeaderFile, MetricFile | XMLParent | swap |  |
-| files2.cpp | library-tests/files/files2.cpp | CppFile, MetricFile | C++, XMLParent | g | x, y |
+| c.c | library-tests/files/c.c | CFile, MetricFile | C |  |  |
+| files1.cpp | library-tests/files/files1.cpp | CppFile, MetricFile | C++ | swap | t |
+| files1.h | library-tests/files/files1.h | HeaderFile, MetricFile |  | swap |  |
+| files2.cpp | library-tests/files/files2.cpp | CppFile, MetricFile | C++ | g | x, y |