Add empty parameter pack regression test

Extend the unused static variable query test with a C++20 variadic parameter pack instantiated with zero and one arguments. This captures the empty-pack restrictionPack finding reported by issue #22931.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jketema

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedStaticVariables/UnusedStaticVariables.expected

@@ -2,3 +2,4 @@
 | test.cpp:8:12:8:21 | staticVar6 | Static variable staticVar6 is never read. |
 | test.cpp:10:11:10:19 | constVar8 | Static variable constVar8 is never read. |
 | test.cpp:12:12:12:22 | staticVar10 | Static variable staticVar10 is never read. |
+| test.cpp:68:58:68:72 | restrictionPack | Static variable restrictionPack is never read. |

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedStaticVariables/test.cpp

@@ -1,4 +1,4 @@
-// semmle-extractor-options: -std=c++17
+// semmle-extractor-options: -std=c++20
 int globalVar; // GOOD (not static)
 static int staticVar1; // GOOD (used)
 static int staticVar2; // GOOD (used)
@@ -49,3 +49,30 @@ struct WithDestructor
 
 static WithConstructor constructed; // GOOD (constructor has a side effect)
 static WithDestructor destructed; // GOOD (destructor has a side effect)
+
+template <typename From, typename To>
+concept ConvertibleTo = requires(From value) { static_cast<To>(value); };
+
+struct Store
+{
+	using LinkAddressType = unsigned;
+	struct LinkType
+	{
+		LinkAddressType restriction;
+	};
+};
+
+template <typename TStore>
+static auto All(
+	const TStore&,
+	ConvertibleTo<typename TStore::LinkAddressType> auto... restrictionPack) // $ SPURIOUS: Alert
+{
+	typename TStore::LinkType restriction{static_cast<typename TStore::LinkAddressType>(restrictionPack)...};
+	return restriction;
+}
+
+int main()
+{
+	Store store;
+	return All(store).restriction + All(store, 2U).restriction;
+}