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.

1 files changed, 13 insertions, 0 deletions

diff --git a/Tests/AK/TestVariant.cpp b/Tests/AK/TestVariant.cpp
index 37bbf646cd..88a3e5fb7d 100644
--- a/Tests/AK/TestVariant.cpp
+++ b/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>));
+}