AK: Allow AK::Variant::visit to return a value

This changes Variant::visit() to forward the value returned by the
selected visitor invocation. By perfectly forwarding the returned value,
this allows for the visitor to return by value or reference.

Note that all provided visitors must return the same type - the compiler
will otherwise fail with the message: "inconsistent deduction for auto
return type".

AK/Variant.h

@@ -9,6 +9,7 @@
 #include <AK/Array.h>
 #include <AK/BitCast.h>
 #include <AK/StdLibExtras.h>
+#include <AK/TypeList.h>
 
 namespace AK::Detail {
 
@@ -68,24 +69,6 @@ struct Variant<IndexType, InitialIndex, F, Ts...> {
         else
             Variant<IndexType, InitialIndex + 1, Ts...>::copy_(old_id, old_data, new_data);
     }
-
-    template<typename Visitor>
-    static void visit_(IndexType id, void* data, Visitor&& visitor)
-    {
-        if (id == current_index)
-            visitor(*bit_cast<F*>(data));
-        else
-            Variant<IndexType, InitialIndex + 1, Ts...>::visit_(id, data, forward<Visitor>(visitor));
-    }
-
-    template<typename Visitor>
-    static void visit_(IndexType id, const void* data, Visitor&& visitor)
-    {
-        if (id == current_index)
-            visitor(*bit_cast<const F*>(data));
-        else
-            Variant<IndexType, InitialIndex + 1, Ts...>::visit_(id, data, forward<Visitor>(visitor));
-    }
 };
 
 template<typename IndexType, IndexType InitialIndex>
@@ -93,10 +76,23 @@ struct Variant<IndexType, InitialIndex> {
     static void delete_(IndexType, void*) { }
     static void move_(IndexType, void*, void*) { }
     static void copy_(IndexType, const void*, void*) { }
-    template<typename Visitor>
+};
-    static void visit_(IndexType, void*, Visitor&&) { }
+
-    template<typename Visitor>
+template<typename IndexType, typename... Ts>
-    static void visit_(IndexType, const void*, Visitor&&) { }
+struct VisitImpl {
+    template<typename Visitor, IndexType CurrentIndex = 0>
+    static constexpr inline decltype(auto) visit(IndexType id, const void* data, Visitor&& visitor) requires(CurrentIndex < sizeof...(Ts))
+    {
+        using T = typename TypeList<Ts...>::template Type<CurrentIndex>;
+
+        if (id == CurrentIndex)
+            return visitor(*bit_cast<T*>(data));
+
+        if constexpr ((CurrentIndex + 1) < sizeof...(Ts))
+            return visit<Visitor, CurrentIndex + 1>(id, data, forward<Visitor>(visitor));
+        else
+            VERIFY_NOT_REACHED();
+    }
 };
 
 struct VariantNoClearTag {
@@ -310,17 +306,17 @@ public:
     }
 
     template<typename... Fs>
-    void visit(Fs&&... functions)
+    decltype(auto) visit(Fs&&... functions)
     {
         Visitor<Fs...> visitor { forward<Fs>(functions)... };
-        Helper::visit_(m_index, m_data, visitor);
+        return VisitHelper::visit(m_index, m_data, move(visitor));
     }
 
     template<typename... Fs>
-    void visit(Fs&&... functions) const
+    decltype(auto) visit(Fs&&... functions) const
     {
         Visitor<Fs...> visitor { forward<Fs>(functions)... };
-        Helper::visit_(m_index, m_data, visitor);
+        return VisitHelper::visit(m_index, m_data, move(visitor));
     }
 
     template<typename... NewTs>
@@ -357,6 +353,7 @@ private:
     static constexpr auto data_size = integer_sequence_generate_array<size_t>(0, IntegerSequence<size_t, sizeof(Ts)...>()).max();
     static constexpr auto data_alignment = integer_sequence_generate_array<size_t>(0, IntegerSequence<size_t, alignof(Ts)...>()).max();
     using Helper = Detail::Variant<IndexType, 0, Ts...>;
+    using VisitHelper = Detail::VisitImpl<IndexType, Ts...>;
 
     explicit Variant(IndexType index, Detail::VariantConstructTag)
         : Detail::MergeAndDeduplicatePacks<Detail::VariantConstructors<Ts, Variant<Ts...>>...>()
@@ -367,7 +364,7 @@ private:
     template<typename... Fs>
     struct Visitor : Fs... {
         Visitor(Fs&&... args)
-            : Fs(args)...
+            : Fs(forward<Fs>(args))...
         {
         }
 

Tests/AK/TestVariant.cpp

@@ -6,8 +6,16 @@
 
 #include <LibTest/TestSuite.h>
 
+#include <AK/RefPtr.h>
 #include <AK/Variant.h>
 
+namespace {
+
+struct Object : public RefCounted<Object> {
+};
+
+}
+
 TEST_CASE(basic)
 {
     Variant<int, String> the_value { 42 };
@@ -117,3 +125,50 @@ TEST_CASE(duplicated_types)
     EXPECT(its_just_an_int.has<int>());
     EXPECT_EQ(its_just_an_int.get<int>(), 42);
 }
+
+TEST_CASE(return_values)
+{
+    using MyVariant = Variant<int, String, float>;
+    {
+        MyVariant the_value { 42.0f };
+
+        float value = the_value.visit(
+            [&](const int&) { return 1.0f; },
+            [&](const String&) { return 2.0f; },
+            [&](const float& f) { return f; });
+        EXPECT_EQ(value, 42.0f);
+    }
+    {
+        MyVariant the_value { 42 };
+
+        int value = the_value.visit(
+            [&](int& i) { return i; },
+            [&](String&) { return 2; },
+            [&](float&) { return 3; });
+        EXPECT_EQ(value, 42);
+    }
+    {
+        const MyVariant the_value { "str" };
+
+        String value = the_value.visit(
+            [&](const int&) { return String { "wrong" }; },
+            [&](const String& s) { return s; },
+            [&](const float&) { return String { "wrong" }; });
+        EXPECT_EQ(value, "str");
+    }
+}
+
+TEST_CASE(return_values_by_reference)
+{
+    auto ref = adopt_ref_if_nonnull(new Object());
+    Variant<int, String, float> the_value { 42.0f };
+
+    auto& value = the_value.visit(
+        [&](const int&) -> RefPtr<Object>& { return ref; },
+        [&](const String&) -> RefPtr<Object>& { return ref; },
+        [&](const float&) -> RefPtr<Object>& { return ref; });
+
+    EXPECT_EQ(ref, value);
+    EXPECT_EQ(ref->ref_count(), 1u);
+    EXPECT_EQ(value->ref_count(), 1u);
+}