M0-1-4: Exclude constexpr variables whose constant is used in a template argument

change_notes/2022-11-02-m0-1-4-single-use-with-templates.md

@@ -0,0 +1,2 @@
+ - `M0-1-4` - `SingleUseMemberPODVariable.ql`
+   - Reduce false positives by excluding any constexpr variable whose constant value is used as an argument to a template.

cpp/autosar/src/rules/M0-1-4/SingleUsePODVariable.qll

@@ -4,6 +4,12 @@ import cpp
 import codingstandards.cpp.TrivialType
 import codingstandards.cpp.deadcode.UnusedVariables
 
+/** Gets the constant value of a constexpr variable. */
+private string getConstExprValue(Variable v) {
+  result = v.getInitializer().getExpr().getValue() and
+  v.isConstexpr()
+}
+
 /** Gets a "use" count according to rule M0-1-4. */
 int getUseCount(Variable v) {
   exists(int initializers |
@@ -12,7 +18,14 @@ int getUseCount(Variable v) {
     result =
       initializers +
         count(VariableAccess access | access = v.getAnAccess() and not access.isCompilerGenerated())
-        + count(UserProvidedConstructorFieldInit cfi | cfi.getTarget() = v)
+        + count(UserProvidedConstructorFieldInit cfi | cfi.getTarget() = v) +
+        // For constexpr variables used as template arguments, we don't see accesses (just the
+        // appropriate literals). We therefore take a conservative approach and count the number of
+        // template instantiations that use the given constant, and consider each one to be a use
+        // of the variable
+        count(ClassTemplateInstantiation cti |
+          cti.getTemplateArgument(_).(Expr).getValue() = getConstExprValue(v)
+        )
   )
 }
 

cpp/autosar/test/rules/M0-1-4/SingleUseMemberPODVariable.expected

@@ -1,3 +1,4 @@
+| test.cpp:36:24:36:29 | unused | Member POD variable unused in C1 is only $@. | test.cpp:36:31:36:31 | initializer for unused | used once |
 | test_global_or_namespace.cpp:16:7:16:7 | x | Member POD variable x in GA is only $@. | test_global_or_namespace.cpp:38:6:38:6 | x | used once |
 | test_global_or_namespace.cpp:54:7:54:7 | x | Member POD variable x in N1A is only $@. | test_global_or_namespace.cpp:76:6:76:6 | x | used once |
 | test_member.cpp:5:7:5:8 | m2 | Member POD variable m2 in A is only $@. | test_member.cpp:9:21:9:25 | constructor init of field m2 | used once |

cpp/autosar/test/rules/M0-1-4/test.cpp

@@ -1,5 +1,5 @@
 /** Test cases for `SingleUseLocalPODVariable.ql` */
-
+#include <array>
 class A {};
 
 class B {
@@ -30,4 +30,12 @@ void test_templates() {
   f1<B>(); // Triggers a NON_COMPLIANT case in f1(), because B is a POD type
   f1<C>(); // Does not trigger a NON_COMPLIANT case in f1(), because C is not a
            // POD type
-}
+}
+
+class C1 {
+  static constexpr int unused{1}; // NON_COMPLIANT
+  static constexpr int used{2};   // COMPLIANT
+  int test_use() { return used; }
+  static constexpr int size{3};               // COMPLIANT[FALSE_POSITIVE]
+  std::array<bool, size> array{false, false}; // size is used here
+};