C++: Add support for getting literals in using declarations

cpp/ql/lib/change-notes/2025-05-28-using-template.md

@@ -0,0 +1,4 @@
+---
+category: feature
+---
+* Added a predicate `getReferencedMember` to `UsingDeclarationEntry`, which yields a member depending on a type template parameter.

cpp/ql/lib/semmle/code/cpp/Namespace.qll

@@ -174,7 +174,27 @@ class UsingDeclarationEntry extends UsingEntry {
    */
   Declaration getDeclaration() { usings(underlyingElement(this), unresolveElement(result), _, _) }
 
-  override string toString() { result = "using " + this.getDeclaration().getDescription() }
+  /**
+   * Gets the member that is referenced by this using declaration, where the member depends on a
+   * type template parameter.
+   *
+   * For example:
+   * ```
+   * template <typename T>
+   * class A {
+   *   using T::m;
+   * };
+   * ```
+   * Here, `getReferencedMember()` yields the member `m` of `T`. Observe that,
+   * as `T` is not instantiated, `m` is represented by a `Literal` and not
+   * a `Declaration`.
+   */
+  Literal getReferencedMember() { usings(underlyingElement(this), unresolveElement(result), _, _) }
+
+  override string toString() {
+    result = "using " + this.getDeclaration().getDescription() or
+    result = "using " + this.getReferencedMember()
+  }
 }
 
 /**

cpp/ql/test/library-tests/comments/binding/commentBinding.expected

@@ -9,3 +9,6 @@
 | multi.c:5:27:5:36 | // Multi 3 | declaration of multi3 |
 | templates.cpp:3:3:3:8 | // Foo | declaration of foo |
 | templates.cpp:7:3:7:8 | // Bar | definition of bar |
+| templates.cpp:16:3:16:20 | // using T::member | using member |
+| templates.cpp:19:3:19:28 | // using T::nested::member | using member |
+| templates.cpp:25:3:25:20 | // using T::member | using member |

cpp/ql/test/library-tests/comments/binding/templates.cpp

@@ -10,3 +10,18 @@ class Cl {
   }
 };
 
+
+template <typename T>
+class Derived : public T {
+  // using T::member
+  using T::member;
+
+  // using T::nested::member
+  using T::nested::member;
+};
+
+template <typename T>
+class Base {
+  // using T::member
+  using T::member;
+};