AK: Specialize TypeList for Variant types

This allows callers to use the following semantics:

    using MyVariant = Variant<Empty, int>;

    template<typename T>
    size_t size() { return TypeList<T>::size; }

    auto s = size<MyVariant>();

This will be needed for an upcoming IPC change, which will result in us
knowing the Variant type, but not the underlying variadic types that the
Variant holds.

AK/Variant.h

@@ -226,7 +226,7 @@ template<NotLvalueReference... Ts>
 struct Variant
     : public Detail::MergeAndDeduplicatePacks<Detail::VariantConstructors<Ts, Variant<Ts...>>...> {
 public:
-    using IndexType = Conditional<sizeof...(Ts) < 255, u8, size_t>; // Note: size+1 reserved for internal value checks
+    using IndexType = Conditional<(sizeof...(Ts) < 255), u8, size_t>; // Note: size+1 reserved for internal value checks
 private:
     static constexpr IndexType invalid_index = sizeof...(Ts);
 
@@ -518,6 +518,9 @@ private:
     IndexType m_index;
 };
 
+template<typename... Ts>
+struct TypeList<Variant<Ts...>> : TypeList<Ts...> { };
+
 }
 
 #if USING_AK_GLOBALLY

Tests/AK/TestVariant.cpp

@@ -260,3 +260,16 @@ TEST_CASE(default_empty)
     EXPECT(my_variant.has<Empty>());
     EXPECT(!my_variant.has<int>());
 }
+
+TEST_CASE(type_list_specialization)
+{
+    EXPECT_EQ((TypeList<Variant<Empty>>::size), 1u);
+    EXPECT_EQ((TypeList<Variant<Empty, int>>::size), 2u);
+    EXPECT_EQ((TypeList<Variant<Empty, int, String>>::size), 3u);
+
+    using MyVariant = Variant<Empty, int, String>;
+    using MyList = TypeList<MyVariant>;
+    EXPECT((IsSame<typename MyList::template Type<0>, Empty>));
+    EXPECT((IsSame<typename MyList::template Type<1>, int>));
+    EXPECT((IsSame<typename MyList::template Type<2>, String>));
+}