tag_invoke does not work for extension of std::array

[pmconrad]

Version of Boost

I'm using the standalone version tagged as 1.75.0 edbf866

Steps necessary to reproduce the problem

#define BOOST_JSON_STANDALONE

#include <boost/json/src.hpp>
#include <boost/json/parse.hpp>
#include <boost/json/serialize.hpp>
#include <boost/json/value.hpp>
#include <boost/json/value_from.hpp>
#include <boost/json/value_to.hpp>

#include <iostream>
#include <sstream>

namespace test {

template<std::size_t N>
class zeroed_array : public std::array<unsigned char, N> {
public:
    zeroed_array() : std::array<unsigned char, N>({0}) {}
};

template<std::size_t N>
void tag_invoke(const boost::json::value_from_tag&, boost::json::value& v, const zeroed_array<N>& a) {
    v = "hello world";
}

template<std::size_t N>
zeroed_array<N> tag_invoke(const boost::json::value_to_tag<zeroed_array<N>>&, const boost::json::value& v) {
    zeroed_array<N> res;
    res[0] = '1';
    return res;
}

}

int main() {
    test::zeroed_array<5> tmp;
    tmp[0] = '2';
    std::cout << boost::json::value_from(tmp) << "\n";
    tmp = boost::json::value_to<test::zeroed_array<5>>(boost::json::parse("\"hello world\""));
    std::cout << tmp[0] << "\n";
}

Put the above code into /tmp/test.cpp and compile with

clang++ --std=c++17 -o /tmp/test -I boost-json/include/ /tmp/test.cpp 

Then run /tmp/test. Actual output is

[50,0,0,0,0]
1

Expected output is

hello world
1

For some reason, template<std::size_t N> void tag_invoke(const boost::json::value_from_tag&, boost::json::value& v, const zeroed_array<N>& a) is not used, but the generic implementation is used instead. I haven't been able to figure out why. Interestingly, the other tag_invoke (with value_to_tag) is being used, as the second line of output shows.

Since value_from_generic is not specific to std::array, I expect that similar problems occur for other base classes.

All relevant compiler information

> clang++ --version
clang version 9.0.1 
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

Same with

> g++-9 --version
g++-9 (SUSE Linux) 9.3.1 20200406 [revision 6db837a5288ee3ca5ec504fbd5a765817e556ac2]
Copyright (C) 2019 Free Software Foundation, Inc.

[vinniefalco]

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