A0-1-6: Handle alias templates

alias templates are not properly handled by the extractor, so we provide
special case handling that ensures alias template instantiations are
correctly associated with alias templates.

alias templates are not represented using the same template
infrastructure as other templates. Instead, each instantiation is a copy
of the template type, with no specified location. We use this to
identify templates with at least one instantiation. We assume an
instantiated template is used, as the only way it would be instantiated
is if it was used.

Author: lcartey

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

@@ -3,6 +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:104:26:104:26 | Y<int> | Type declaration Y<int> is not used. |
-| test.cpp:107:29:107:29 | Z | Type declaration Z 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

@@ -101,10 +101,10 @@ 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[FALSE_POSITIVE] - used in the test case below
+template <class T> class Y {}; // COMPLIANT - used in the test case below
 
 // Alias templates
-template <typename T> using Z = Y<T>; // COMPLIANT[FALSE_POSITIVE] - used below
+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() {

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()
+    )
   )
 }