Fixes problem of working on structs with only a single element

include/reflection-cpp/reflection.hpp

@@ -715,7 +715,7 @@ constexpr void EnumerateMembers(Callable&& callable)
 }
 
 template <typename Object, typename Callable>
-    requires std::same_as<void, std::invoke_result_t<Callable, std::string, MemberTypeOf<1, Object>>>
+    requires std::same_as<void, std::invoke_result_t<Callable, std::string, MemberTypeOf<0, Object>>>
 void CallOnMembers(Object& object, Callable&& callable)
 {
     EnumerateMembers<Object>(object,
@@ -752,7 +752,7 @@ constexpr ResultType FoldMembers(ResultType initialValue, Callable const& callab
 ///
 /// @return The result of the fold
 template <typename Object, typename Callable, typename ResultType>
-    requires std::same_as<ResultType, std::invoke_result_t<Callable, std::string, MemberTypeOf<1, Object>, ResultType>>
+    requires std::same_as<ResultType, std::invoke_result_t<Callable, std::string, MemberTypeOf<0, Object>, ResultType>>
 constexpr ResultType FoldMembers(Object& object, ResultType initialValue, Callable const& callable)
 {
     // clang-format off

test-reflection-cpp.cpp

@@ -30,6 +30,11 @@ enum Color
     Blue
 };
 
+struct SingleValueRecord
+{
+    int value;
+};
+
 TEST_CASE("GetName", "[reflection]")
 {
     auto const enumValue = Reflection::GetName<Color::Red>();
@@ -38,13 +43,34 @@ TEST_CASE("GetName", "[reflection]")
     auto const enumValue2 = Reflection::GetName<Color::Green>();
     CHECK(enumValue2 == "Green");
 
-    auto const person = Person { "John Doe", "[email protected]", 42 };
     auto const memberName1 = Reflection::GetName<&Person::email>();
     CHECK(memberName1 == "email");
+
+    auto const singleValueField = Reflection::GetName<&SingleValueRecord::value>();
+    CHECK(singleValueField == "value");
+}
+
+TEST_CASE("single value record", "[reflection]")
+{
+    static_assert(Reflection::CountMembers<SingleValueRecord> == 1);
+
+    auto const s = SingleValueRecord { 42 };
+    auto const t = Reflection::ToTuple(s);
+
+    CHECK(std::get<0>(t) == 42);
+    CHECK(Reflection::GetMemberAt<0>(s) == 42);
+
+    Reflection::CallOnMembers(s, [](auto&& name, auto&& value) {
+        CHECK(name == "value");
+        CHECK(value == 42);
+    });
 }
 
 TEST_CASE("core", "[reflection]")
 {
+    auto s = SingleValueRecord { 42 };
+    CHECK(Reflection::Inspect(s) == "value=42");
+
     auto p = Person { "John Doe", "[email protected]", 42 };
     auto const result = Reflection::Inspect(p);
     CHECK(result == R"(name="John Doe" email="[email protected]" age=42)");