C++ name lookup inconsistency for template/overloaded functions - c++

Question

I'm building a minimal binary parser/serializer for one of my projects and I got this error that seems inconsistent at first. I have two top functions I want to expose T parse<T>(bytes) and bytevec serialize<T>(T const&) and from these two templated functions I delegate the serialization to overloaded functions based on the specific type provided. The problem is that the parsing side works fine, but the serialization side doesn't compile because it cannot see the overloaded function if it is defined after the top serialize function.
using bytevec = std::vector<uint8_t>;
using bytes = std::span<const uint8_t>;
template <typename T>
bytevec serialize(T const& data) {
bytevec b{};
serialize(data, b); // error: no matching function for call to 'serialize(const int&, bytevec&)
return b;
}
void serialize(int data, bytevec& b) {
b.push_back(0xff);
b.push_back(0xaa);
}
template <typename T>
struct tag {};
template <typename T>
T parse(bytes b) {
return parse(b, tag<T>{}); // fine
}
int parse(bytes b, tag<int>) { return b[1]; }
void test() {
static std::array<uint8_t, 2> data{0xff, 0xaa};
auto res1 = parse<int>(std::span(data));
auto res2 = serialize(res1);
}
Link to compiler explorer
Why is it ok for the top parse function to call an overloaded function defined later (int parse(bytes, tag<int>) and for the serialize function the overloaded function needs to be defined before in order to compile?
Thank you

Answer 1

ADL doesn't work for built-in types such as int.
For
template <typename T>
bytevec serialize(T const& data) {
std::vector<uint8_t> b{};
serialize(data, b);
return b;
}
With T=int
serialize(data, b); uses previous declaration, and can be looked-up in same namespace than std::vector<uint8_t> (so std). But no previous declarations and no std::serialize.
For
template <typename T>
T parse(bytes b) {
return parse(b, tag<T>{});
}
with T=int
parse(b, tag<int>{}) uses previous declaration, and can be looked-up in same namespace than tag<int> (so the global one) and std::span<const uint8_t> (so std). No previous declaration nor std::parse, but ::parse(int, tag<int>{}) exist.