C++: Fix characteristic predicate of XMLParent.

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

@@ -0,0 +1,4 @@
+| 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 |

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

@@ -0,0 +1,18 @@
+import cpp
+
+string describe(File f) {
+  f.compiledAsC() and
+  result = "C"
+  or
+  f.compiledAsCpp() and
+  result = "C++"
+  or
+  f instanceof XMLParent and
+  result = "XMLParent" // regression tests a bug in the characteristic predicate of XMLParent
+}
+
+from File f
+where f.toString() != ""
+select f.toString(), f.getRelativePath(), concat(f.getAQlClass().toString(), ", "),
+  concat(describe(f), ", "), concat(f.getATopLevelDeclaration().toString(), ", "),
+  concat(LocalVariable v | f.getADeclaration() = v | v.toString(), ", ")