invalid use of incomplete type (boost::optional) / compile error

Description

Hi there,

I get some compiler errors, which I unfortunately don't understand in detail. See the small minimal example. Probably it's a compiler bug, because it compiles with gcc 11+ and clang11. It also compiles with gcc 10 when using -std=c++17.

The code also compiles fine, if I comment out the "to_json" function or make the Dummy constructor non-explicit. I don't understand why in the gcc10 setup with c++11 standard, the to_json function gets inspected at all. How can it matter if this function exists or not?

Thank you for your comments!

Reproduction steps

See also https://godbolt.org/z/1Whjn8nKn

Expected vs. actual results

It should compile, but it doesn't.

Minimal code example

#include <boost/optional.hpp>

#include <nlohmann/json.hpp>

class Dummy {
    explicit Dummy(const nlohmann::json& data) {
    }
};

class Holder {
    boost::optional<Dummy> d;

public:
};

void to_json(nlohmann::json& j, const Holder& h) {
}

int main(int argc, char* argv[]) {
    Holder h;
    return 0;
}

Error messages

In file included from <source>:3:
/opt/compiler-explorer/libs/nlohmann_json/v3.6.0/single_include/nlohmann/json.hpp: In substitution of 'template<class Expected, template<class ...> class Op, class ... Args> using is_detected_exact = std::is_same<Expected, typename nlohmann::detail::detector<nlohmann::detail::nonesuch, void, Op, Args ...>::type> [with Expected = void; Op = nlohmann::detail::to_json_function; Args = {nlohmann::adl_serializer<Dummy, void>, nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>&, Dummy}]':
/opt/compiler-explorer/libs/nlohmann_json/v3.6.0/single_include/nlohmann/json.hpp:986:13:   required from 'constexpr const bool nlohmann::detail::has_to_json<nlohmann::basic_json<>, Dummy, void>::value'
/opt/compiler-explorer/libs/nlohmann_json/v3.6.0/single_include/nlohmann/json.hpp:1204:53:   required from 'constexpr const bool nlohmann::detail::is_compatible_type_impl<nlohmann::basic_json<>, Dummy, void>::value'
/opt/compiler-explorer/libs/nlohmann_json/v3.6.0/single_include/nlohmann/json.hpp:14016:55:   required by substitution of 'template<class CompatibleType, class U, typename std::enable_if<((! nlohmann::detail::is_basic_json<T>::value) && nlohmann::detail::is_compatible_type<nlohmann::basic_json<>, U>::value), int>::type <anonymous> > nlohmann::basic_json<>::basic_json(CompatibleType&&) [with CompatibleType = Dummy; U = Dummy; typename std::enable_if<((! nlohmann::detail::is_basic_json<T>::value) && nlohmann::detail::is_compatible_type<nlohmann::basic_json<>, U>::value), int>::type <anonymous> = <missing>]'
/opt/compiler-explorer/libs/boost_1_79_0/boost/type_traits/is_constructible.hpp:37:65:   required by substitution of 'template<class T, class Arg, class> static boost::type_traits::yes_type boost::detail::is_constructible_imp::test1(int) [with T = Dummy; Arg = Dummy; <template-parameter-1-3> = <missing>]'
/opt/compiler-explorer/libs/boost_1_79_0/boost/type_traits/is_constructible.hpp:54:185:   required from 'struct boost::is_constructible<Dummy, Dummy>'
/opt/compiler-explorer/libs/boost_1_79_0/boost/optional/optional.hpp:816:8:   [ skipping 10 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]
/opt/compiler-explorer/libs/nlohmann_json/v3.6.0/single_include/nlohmann/json.hpp:14016:55:   required by substitution of 'template<class CompatibleType, class U, typename std::enable_if<((! nlohmann::detail::is_basic_json<T>::value) && nlohmann::detail::is_compatible_type<nlohmann::basic_json<>, U>::value), int>::type <anonymous> > nlohmann::basic_json<>::basic_json(CompatibleType&&) [with CompatibleType = const Dummy&; U = Dummy; typename std::enable_if<((! nlohmann::detail::is_basic_json<T>::value) && nlohmann::detail::is_compatible_type<nlohmann::basic_json<>, U>::value), int>::type <anonymous> = <missing>]'
/opt/compiler-explorer/libs/boost_1_79_0/boost/type_traits/is_constructible.hpp:37:65:   required by substitution of 'template<class T, class Arg, class> static boost::type_traits::yes_type boost::detail::is_constructible_imp::test1(int) [with T = Dummy; Arg = const Dummy&; <template-parameter-1-3> = <missing>]'
/opt/compiler-explorer/libs/boost_1_79_0/boost/type_traits/is_constructible.hpp:54:185:   required from 'struct boost::is_constructible<Dummy, const Dummy&>'
/opt/compiler-explorer/libs/boost_1_79_0/boost/optional/optional.hpp:816:8:   required from 'struct boost::optional_detail::is_optional_constructible<Dummy, const Dummy&>'
/opt/compiler-explorer/libs/boost_1_79_0/boost/core/enable_if.hpp:41:10:   required from 'struct boost::enable_if<boost::optional_detail::is_optional_constructible<Dummy, const Dummy&>, bool>'
/opt/compiler-explorer/libs/boost_1_79_0/boost/optional/optional.hpp:964:14:   required by substitution of 'template<class U> boost::optional<Dummy>::optional(const boost::optional<T>&, typename boost::enable_if<boost::optional_detail::is_optional_constructible<Dummy, const U&>, bool>::type) [with U = Dummy]'
<source>:20:12:   required from here
/opt/compiler-explorer/libs/nlohmann_json/v3.6.0/single_include/nlohmann/json.hpp:802:7: error: invalid use of incomplete type 'struct nlohmann::detail::detector<nlohmann::detail::nonesuch, void, nlohmann::detail::to_json_function, nlohmann::adl_serializer<Dummy, void>, nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>&, Dummy>'
  802 | using is_detected_exact = std::is_same<Expected, detected_t<Op, Args...>>;
      |       ^~~~~~~~~~~~~~~~~
/opt/compiler-explorer/libs/nlohmann_json/v3.6.0/single_include/nlohmann/json.hpp:776:8: note: declaration of 'struct nlohmann::detail::detector<nlohmann::detail::nonesuch, void, nlohmann::detail::to_json_function, nlohmann::adl_serializer<Dummy, void>, nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>&, Dummy>'
  776 | struct detector
      |        ^~~~~~~~
Compiler returned: 1

Compiler and operating system

gcc 10, linux,

Library version

3.10.3

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.

Reproduce the minimal example from the issue with GCC 10 in C++11, using boost::optional and nlohmann/json.hpp, then compare it with GCC 11+, Clang 11, and C++17. Use the Compiler Explorer example and the reported template-instantiation trace to identify why to_json triggers the incomplete-type error; done means the example compiles without requiring a non-explicit Dummy constructor.