[C++20][modules] Combining modules, concepts (and a JSON library) leads to a linkage error

[AlbertSSj]

I have a small module (A) that defines a template function on objects that can be casted to an nlohmann::json object, and a concept to enable the operation. I have a separate module (B) to use the operation.
When used in a module, this leads to a linkage error. The linker seems to believe that the symbols for some of the JSON operation have to be provided by module A.

What is interesting is that by forcing module A to generate the symbols, by creating an exported function that does a bunch of JSON conversions, the linkage can complete successfully. So for now I'm adding one by one the JSON conversions that are required by B in A.

I don't even need to use my function to cause the issue, just defining a JSON object is enough.

Below is a set of files that can reproduce the problem:

module_a.cppm

module;

#include <nlohmann/json.hpp>

export module module_a;

namespace module_a {

template <typename T>
concept FromOrderedJson = requires(T t) {
    { static_cast<nlohmann::ordered_json>(t) };
};

template <typename T>
concept FromJson = requires(T t) {
    { static_cast<nlohmann::json>(t) };
};

export template <typename T>
    requires(FromOrderedJson<T>)
std::string MakeString(T value)
{
    return nlohmann::ordered_json(value);
}

export template <typename T>
    requires(FromJson<T>)
std::string MakeString(T value)
{
    return nlohmann::json(value);
}

} // namespace module_a

module_b.cpp

module;

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

export module module_b;
import module_a;

namespace {
struct MyJsonData
{
    std::string a;
    int b;
    bool c;
};

void to_json(nlohmann::json& j, const MyJsonData& p)
{
    j["a"] = p.a;
    j["b"] = p.b;
    j["c"] = nlohmann::json(p.c);
}
} // namespace

int main(int argc, char* argv[])
{
    MyJsonData value;
    auto x = nlohmann::json(value);
}

CMakeLists.txt

add_library(module_a SHARED)
target_sources(
    module_a
    PUBLIC FILE_SET modules TYPE CXX_MODULES FILES
        module_a.cppm
)
target_compile_options(
    module_a
    #PUBLIC
    #-fexperimental-modules-reduced-bmi
)
target_link_libraries(
    module_a
)
add_executable(module_b)
target_sources(
    module_b
    PUBLIC FILE_SET modules TYPE CXX_MODULES FILES
        module_b.cpp
)
target_link_libraries(
    module_b
    PUBLIC
    module_a
)

The error is this:

/usr/bin/ld: module_b.cpp.o: in function `_ZN8nlohmann10basic_jsonISt3mapSt6vectorNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEblmdSaNS_14adl_serializerES2_IhSaIhEEEC2IRKbbTnNSt9enable_ifIXaantsr6detail13is_basic_jsonIT0_EE5valuesr6detail18is_compatible_typeISC_SH_EE5valueEiE4typeELi0EEEOT_':
module_b.cpp:17931:(.text._ZN8nlohmann10basic_jsonISt3mapSt6vectorNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEblmdSaNS_14adl_serializerES2_IhSaIhEEEC2IRKbbTnNSt9enable_ifIXaantsr6detail13is_basic_jsonIT0_EE5valuesr6detail18is_compatible_typeISC_SH_EE5valueEiE4typeELi0EEEOT_[_ZN8nlohmann10basic_jsonISt3mapSt6vectorNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEblmdSaNS_14adl_serializerES2_IhSaIhEEEC2IRKbbTnNSt9enable_ifIXaantsr6detail13is_basic_jsonIT0_EE5valuesr6detail18is_compatible_typeISC_SH_EE5valueEiE4typeELi0EEEOT_]+0x34): undefined reference to `decltype ((nlohmann::(anonymous namespace)::to_json({parm#1}, (std::forward<bool const&>)({parm#2}))),((void)())) nlohmann::adl_serializer<bool, void>::to_json<nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer, std::vector<unsigned char, std::allocator<unsigned char> > >, bool const&>(nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer, std::vector<unsigned char, std::allocator<unsigned char> > >&, bool const&)'

Which basically means that the symbol for the JSON conversion from bool is missing (I think).

In case it helps, I'm using clang version 19.1.6 and cmake version 3.28.3.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: None (AlbertSSj)

I have a small module (A) that defines a template function on objects that can be casted to an nlohmann::json object, and a concept to enable the operation. I have a separate module (B) to use the operation. When used in a module, this leads to a linkage error. The linker seems to believe that the symbols for some of the JSON operation have to be provided by module A.

What is interesting is that by forcing module A to generate the symbols, by creating an exported function that does a bunch of JSON conversions, the linkage can complete successfully. So for now I'm adding one by one the JSON conversions that are required by B in A.

I don't even need to use my function to cause the issue, just defining a JSON object is enough.

Below is a set of files that can reproduce the problem:

module_a.cppm

module;

#include <nlohmann/json.hpp>

export module module_a;

namespace module_a {

template <typename T>
concept FromOrderedJson = requires(T t) {
    { static_cast<nlohmann::ordered_json>(t) };
};

template <typename T>
concept FromJson = requires(T t) {
    { static_cast<nlohmann::json>(t) };
};

export template <typename T>
    requires(FromOrderedJson<T>)
std::string MakeString(T value)
{
    return nlohmann::ordered_json(value);
}

export template <typename T>
    requires(FromJson<T>)
std::string MakeString(T value)
{
    return nlohmann::json(value);
}

} // namespace module_a

module_b.cpp

module;

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

export module module_b;
import module_a;

namespace {
struct MyJsonData
{
    std::string a;
    int b;
    bool c;
};

void to_json(nlohmann::json& j, const MyJsonData& p)
{
    j["a"] = p.a;
    j["b"] = p.b;
    j["c"] = nlohmann::json(p.c);
}
} // namespace

int main(int argc, char* argv[])
{
    MyJsonData value;
    auto x = nlohmann::json(value);
}

CMakeLists.txt

add_library(module_a SHARED)
target_sources(
    module_a
    PUBLIC FILE_SET modules TYPE CXX_MODULES FILES
        module_a.cppm
)
target_compile_options(
    module_a
    #PUBLIC
    #-fexperimental-modules-reduced-bmi
)
target_link_libraries(
    module_a
)
add_executable(module_b)
target_sources(
    module_b
    PUBLIC FILE_SET modules TYPE CXX_MODULES FILES
        module_b.cpp
)
target_link_libraries(
    module_b
    PUBLIC
    module_a
)

The error is this:

/usr/bin/ld: module_b.cpp.o: in function `_ZN8nlohmann10basic_jsonISt3mapSt6vectorNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEblmdSaNS_14adl_serializerES2_IhSaIhEEEC2IRKbbTnNSt9enable_ifIXaantsr6detail13is_basic_jsonIT0_EE5valuesr6detail18is_compatible_typeISC_SH_EE5valueEiE4typeELi0EEEOT_':
module_b.cpp:17931:(.text._ZN8nlohmann10basic_jsonISt3mapSt6vectorNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEblmdSaNS_14adl_serializerES2_IhSaIhEEEC2IRKbbTnNSt9enable_ifIXaantsr6detail13is_basic_jsonIT0_EE5valuesr6detail18is_compatible_typeISC_SH_EE5valueEiE4typeELi0EEEOT_[_ZN8nlohmann10basic_jsonISt3mapSt6vectorNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEblmdSaNS_14adl_serializerES2_IhSaIhEEEC2IRKbbTnNSt9enable_ifIXaantsr6detail13is_basic_jsonIT0_EE5valuesr6detail18is_compatible_typeISC_SH_EE5valueEiE4typeELi0EEEOT_]+0x34): undefined reference to `decltype ((nlohmann::(anonymous namespace)::to_json({parm#<!-- -->1}, (std::forward&lt;bool const&amp;&gt;)({parm#<!-- -->2}))),((void)())) nlohmann::adl_serializer&lt;bool, void&gt;::to_json&lt;nlohmann::basic_json&lt;std::map, std::vector, std::__cxx11::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt; &gt;, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer, std::vector&lt;unsigned char, std::allocator&lt;unsigned char&gt; &gt; &gt;, bool const&amp;&gt;(nlohmann::basic_json&lt;std::map, std::vector, std::__cxx11::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt; &gt;, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer, std::vector&lt;unsigned char, std::allocator&lt;unsigned char&gt; &gt; &gt;&amp;, bool const&amp;)'

Which basically means that the symbol for the JSON conversion from bool is missing (I think).

In case it helps, I'm using clang version 19.1.6 and cmake version 3.28.3.

[ChuanqiXu9]

It will be helpful for me if we can have a reproducer without any dependencies (it will be better if the stl is not needed.)

[AlbertSSj]

@ChuanqiXu9 I can remove stl, however right now I would not know how to recreate the issue without using nlohmann::json, can that stay?

[ChuanqiXu9]

@ChuanqiXu9 I can remove stl, however right now I would not know how to recreate the issue without using nlohmann::json, can that stay?

The nlohmann::json part is slightly harder for me. If you like, you can try to preprocess the reproducer and reduce the preprocessed one further. This is more or less time consuming. Generally it may take me 1~2 hours to do this.