C++: Support x-macros that are #undef'ed in header

This fixes a false positive on https://github.com/zduka/tpp.

Author: jbj

cpp/ql/src/jsf/4.07 Header Files/AV Rule 35.ql

@@ -95,32 +95,55 @@ predicate usesMacro(HeaderFile hf, string macroName) {
   )
 }
 
-/**
- * File `f` both defines and un-defines a macro called `macroName`.
- */
-predicate defUndef(File f, string macroName) {
+/** File `f` defines a macro called `macroName`. */
+predicate definesMacro(File f, string macroName) {
   exists(Macro m |
     m.getFile() = f and
     m.getName() = macroName
-  ) and exists(PreprocessorUndef ud |
+  )
+}
+
+/** File `f` un-defines a macro called `macroName`. */
+predicate undefinesMacro(File f, string macroName) {
+  exists(PreprocessorUndef ud |
     ud.getFile() = f and
     ud.getName() = macroName
   )
 }
 
+/**
+ * File `f` both defines and un-defines a macro called `macroName`.
+ */
+predicate defUndef(File f, string macroName) {
+  definesMacro(f, macroName) and
+  undefinesMacro(f, macroName)
+}
+
 /**
  * Header file `hf` looks like it contains an x-macro.
  * That is, a macro that is used to interpret the
  * data in `hf`, usually defined just before including that file
  * and undefined immediately afterwards.
  */
 predicate hasXMacro(HeaderFile hf) {
+  // Every header that includes `hf` both defines and undefines a macro that's
+  // used in `hf`.
   exists(string macroName |
     usesMacro(hf, macroName) and
     forex(File f | f.getAnIncludedFile() = hf |
       defUndef(f, macroName)
     )
   )
+  or
+  // Every header that includes `hf` defines a macro that's used in `hf`, and
+  // `hf` itself undefines it.
+  exists(string macroName |
+    usesMacro(hf, macroName) and
+    undefinesMacro(hf, macroName) and
+    forex(File f | f.getAnIncludedFile() = hf |
+      definesMacro(f, macroName)
+    )
+  )
 }
 
 from HeaderFile hf, string detail, MaybePreprocessorDirective detail1, MaybePreprocessorDirective detail2

cpp/ql/test/query-tests/jsf/4.07 Header Files/AV Rule 35/AV Rule 35.expected

@@ -1,7 +1,6 @@
 | complexcondition2.h:0:0:0:0 | complexcondition2.h | This header file should contain a header guard to prevent multiple inclusion. | file://:0:0:0:0 |  |  | file://:0:0:0:0 |  |  |
 | complexcondition.h:0:0:0:0 | complexcondition.h | This header file should contain a header guard to prevent multiple inclusion. | file://:0:0:0:0 |  |  | file://:0:0:0:0 |  |  |
 | fundecl.h:0:0:0:0 | fundecl.h | This header file should contain a header guard to prevent multiple inclusion. | file://:0:0:0:0 |  |  | file://:0:0:0:0 |  |  |
-| items3.h:0:0:0:0 | items3.h | This header file should contain a header guard to prevent multiple inclusion. | file://:0:0:0:0 |  |  | file://:0:0:0:0 |  |  |
 | multipleguards.h:0:0:0:0 | multipleguards.h | This header file should contain a header guard to prevent multiple inclusion ($@ matching $@ occurs before the end of the file). | multipleguards.h:7:1:7:6 | #endif | #endif | multipleguards.h:2:1:2:24 | #ifndef MULTIPLEGUARDS_H | #ifndef MULTIPLEGUARDS_H |
 | namespace1.h:0:0:0:0 | namespace1.h | This header file should contain a header guard to prevent multiple inclusion. | file://:0:0:0:0 |  |  | file://:0:0:0:0 |  |  |
 | namespace2.h:0:0:0:0 | namespace2.h | This header file should contain a header guard to prevent multiple inclusion. | file://:0:0:0:0 |  |  | file://:0:0:0:0 |  |  |