feat: support enums in D-Bus serialization and signatures (#416)

tests/unittests/Message_test.cpp

@@ -347,6 +347,24 @@ TEST(AMessage, CanCarryDBusArrayOfNontrivialTypesGivenAsStdSpan)
 }
 #endif
 
+TEST(AMessage, CanCarryAnEnumValue)
+{
+    auto msg = sdbus::createPlainMessage();
+
+    enum class EnumA : int16_t {X = 5} aWritten{EnumA::X};
+    enum EnumB {Y = 11} bWritten{EnumB::Y};
+
+    msg << aWritten << bWritten;
+    msg.seal();
+
+    EnumA aRead{};
+    EnumB bRead{};
+    msg >> aRead >> bRead;
+
+    ASSERT_THAT(aRead, Eq(aWritten));
+    ASSERT_THAT(bRead, Eq(bWritten));
+}
+
 TEST(AMessage, ThrowsWhenDestinationStdArrayIsTooSmallDuringDeserialization)
 {
     auto msg = sdbus::createPlainMessage();

tests/unittests/TypeTraits_test.cpp

@@ -49,6 +49,21 @@ namespace
         static std::string getDBusTypeSignature();
     };
 
+    enum class SomeEnumClass : uint8_t
+    {
+        A, B, C
+    };
+
+    enum struct SomeEnumStruct : int64_t
+    {
+        A, B, C
+    };
+
+    enum SomeClassicEnum
+    {
+        A, B, C
+    };
+
 #define TYPE(...)                                                                       \
     template <>                                                                         \
     std::string Type2DBusTypeSignatureConversion<__VA_ARGS__>::getDBusTypeSignature()   \
@@ -79,8 +94,11 @@ namespace
     TYPE(std::vector<int16_t>)HAS_DBUS_TYPE_SIGNATURE("an")
     TYPE(std::array<int16_t, 3>)HAS_DBUS_TYPE_SIGNATURE("an")
 #if __cplusplus >= 202002L
-    TYPE(std::span<int16_t>)HAS_DBUS_TYPE_SIGNATURE("ao")
+    TYPE(std::span<int16_t>)HAS_DBUS_TYPE_SIGNATURE("an")
 #endif
+    TYPE(SomeEnumClass)HAS_DBUS_TYPE_SIGNATURE("y")
+    TYPE(SomeEnumStruct)HAS_DBUS_TYPE_SIGNATURE("x")
+    TYPE(SomeClassicEnum)HAS_DBUS_TYPE_SIGNATURE("u")
     TYPE(std::map<int32_t, int64_t>)HAS_DBUS_TYPE_SIGNATURE("a{ix}")
     TYPE(std::unordered_map<int32_t, int64_t>)HAS_DBUS_TYPE_SIGNATURE("a{ix}")
     using ComplexType = std::map<
@@ -127,6 +145,9 @@ namespace
 #if __cplusplus >= 202002L
                             , std::span<int16_t>
 #endif
+                            , SomeEnumClass
+                            , SomeEnumStruct
+                            , SomeClassicEnum
                             , std::map<int32_t, int64_t>
                             , std::unordered_map<int32_t, int64_t>
                             , ComplexType