A0-1-6: Do not consider specialized alias templates as unused

change_notes/2022-08-01-a0-1-6-alias-templates.md

@@ -0,0 +1 @@
+- `A0-1-6`: alias templates are now appropriately handled, with alias templates considered used if there exists an instantiation that is used.

cpp/autosar/test/rules/A0-1-6/UnusedTypeDeclarations.expected

@@ -3,3 +3,4 @@
 | test.cpp:10:11:10:11 | D | Type declaration D is not used. |
 | test.cpp:74:11:74:11 | R | Type declaration R is not used. |
 | test.cpp:87:12:87:12 | struct <unnamed> | Type declaration struct <unnamed> is not used. |
+| test.cpp:108:29:108:30 | AA | Type declaration AA is not used. |

cpp/autosar/test/rules/A0-1-6/test.cpp

@@ -99,4 +99,12 @@ void test_nested_struct() {
 
 template <class T> class X { // COMPLIANT - template class never instantiated
   using custom_type = E<T>;  // COMPLIANT - template class never instantiated
-};
+};
+
+template <class T> class Y {}; // COMPLIANT - used in the test case below
+
+// Alias templates
+template <typename T> using Z = Y<T>;  // COMPLIANT - used below
+template <typename T> using AA = Y<T>; // NON_COMPLIANT - never instantiated
+
+void test_alias_template() { Z<int> v; }

cpp/common/src/codingstandards/cpp/TypeUses.qll

@@ -66,8 +66,15 @@ Locatable getATypeUse(Type type) {
 }
 
 private Locatable getATypeUse_i(Type type) {
-  // Restrict to uses within the source checkout root
-  exists(result.getFile().getRelativePath()) and
+  (
+    // Restrict to uses within the source checkout root
+    exists(result.getFile().getRelativePath())
+    or
+    // Unless it's an alias template instantiation, as they do not have correct locations (last
+    // verified in CodeQL CLI 2.7.6)
+    result instanceof UsingAliasTypedefType and
+    not exists(result.getLocation())
+  ) and
   (
     // Used as a variable type
     exists(Variable v | result = v |
@@ -143,5 +150,20 @@ private Locatable getATypeUse_i(Type type) {
     or
     // This is a TemplateClass where one of the specializations is used
     type = used.(ClassTemplateSpecialization).getPrimaryTemplate()
+    or
+    // Alias templates - alias templates and instantiations are not properly captured by the
+    // extractor (last verified in CodeQL CLI 2.7.6). The only distinguishing factor is that
+    // instantiations of alias templates do not have a location.
+    exists(UsingAliasTypedefType template, UsingAliasTypedefType instantiation |
+      // Instantiation is a "use" of the template
+      used = instantiation and
+      type = template and
+      // The template has a location
+      exists(template.getLocation()) and
+      // The instantiation does not
+      not exists(instantiation.getLocation()) and
+      // Template and instantiation both have the same qualified name
+      template.getQualifiedName() = instantiation.getQualifiedName()
+    )
   )
 }