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".

1 files changed, 55 insertions, 0 deletions

diff --git a/Tests/AK/TestVariant.cpp b/Tests/AK/TestVariant.cpp
index 587eae2819..28dc92340c 100644
--- a/Tests/AK/TestVariant.cpp
+++ b/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);
+}