Hidden friend to_json function unexpectedly resolves for shared_ptr

The code below (Goldbolt) compiles and runs (on both gcc and clang) and does what I would hope. But I'm surprised! I expected to have to use an adl_serializer specialisation (as opposed to the hidden friend here) for it to be able to find the to_json/from_json functions, as the Example class is hidden inside a std::shared_ptr.

So the question is, is this expected (and why?), or a bug in the compilers that might break my code when it gets fixed.

#include <memory>
#include <nlohmann/json.hpp>

struct Example {
    int a;
    int b;

    Example(int a, int b) : a(a), b(b){};

    Example(Example&&) = delete;
    Example& operator=(Example&&) = delete;
    Example(Example&) = delete;
    Example& operator=(Example&) = delete;

    friend void to_json(nlohmann::json& j, const std::shared_ptr<Example>& p) {
        j = {{"a", p->a}, {"b", p->b}};
    }

    friend void from_json(const nlohmann::json& j,
                          std::shared_ptr<Example>& p) {
        p = std::make_shared<Example>(j.at("a").get<int>(),
                                       j.at("b").get<int>());
    }
};

int main() {
    auto j = R"({ "a" : 1, "b" : 2 })"_json;

    auto c = j.get<std::shared_ptr<Example>>();

    auto j2 = nlohmann::json(c);
}