Add is() matchers for templates

Author: filipsajdak

include/cpp2util.h

@@ -513,6 +513,33 @@ static std::nullptr_t nonesuch = nullptr;
 //  TODO: Does this really warrant a new synonym? Perhaps "is void" is enough
 using empty = void;
 
+template <template <typename...> class C, typename... Ts>
+constexpr auto is(const C<Ts...>&) -> bool {
+    return true;
+}
+
+#if defined(_MSC_VER)
+    template <template <typename, typename...> class C, typename T>
+    constexpr auto is(const T&) -> bool {
+        return false;
+    }
+#else
+    template <template <typename...> class C, typename T>
+    constexpr auto is(const T&) -> bool {
+        return false;
+    }
+#endif
+
+template <template <typename,auto> class C, typename T, auto V>
+constexpr auto is(const C<T, V>&) -> bool {
+    return true;
+}
+
+template <template <typename,auto> class C, typename T>
+constexpr auto is(const T&) -> bool {
+    return false;
+}
+
 template< typename C, typename X >
 auto is( X const& ) -> bool {
     return false;