VS2026 Insiders, C2678 With C++23 Modules

[ToddThomson]

Description

When using the Module build of nlohmann::json I get the following error:
binary '!=': no operator found which takes a left-hand operand of type 'nullptr' (or there is no acceptable conversion)
This occurs at line 20512 of the current dev branch of json.hpp.

module;
#include <cstdint>
#include <string>
//#include <sstream> // TJT: Note: Without this include, VS2026 will not complile the toJson() 

export module Dnn.Modules.Softmax:Config;

import Dnn.ModuleConfig;
import nlohmann.json;

namespace Mila::Dnn
{
    using json = nlohmann::json;
// Later

json toJson() const
{
    json j;
    j["name"] = name_;
    //j["precision"] = static_cast<int>( precision_ );
    //j["axis"] = axis_;

    return j;
}`

With the included (Another similar module had this include and compiled fine AND tested fine ) there is no compilation error.

I'll look into this more but I am busy for the rest of the day

Reproduction steps

not having the include
#include <sstream>
triggers the compilation error c2678

Expected vs. actual results

The include essentially is providing a needed dependency that is missing in the json.hpp module based code.

Minimal code example

Error messages

Compiler and operating system

Win 11, VS2026

Library version

Dev branch, Module build

Validation

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

[ToddThomson]

Better workaround is

module; 
#include <memory>

Then add your import nlohmann.json

[ToddThomson]

Hi @nlohmann,

Here are the mechanics of the issue. I will investigate further if you require.

In json.hpp, Line 20512, dev branch:

/// helper for exception-safe object creation
template<typename T, typename... Args>
JSON_HEDLEY_RETURNS_NON_NULL
static T* create(Args&& ... args)
{
    AllocatorType<T> alloc;
    using AllocatorTraits = std::allocator_traits<AllocatorType<T>>;

    auto deleter = [&](T * obj)
    {
        AllocatorTraits::deallocate(alloc, obj, 1);
    };
    std::unique_ptr<T, decltype(deleter)> obj(AllocatorTraits::allocate(alloc, 1), deleter);
    AllocatorTraits::construct(alloc, obj.get(), std::forward<Args>(args)...);
    JSON_ASSERT(obj != nullptr);
    return obj.release();
}

the
JSON_ASSERT(obj != nullptr);

requires the function:

_EXPORT_STD template <class _Ty, class _Dx> _NODISCARD _CONSTEXPR23 bool operator==(const unique_ptr<_Ty, _Dx>& _Left, nullptr_t) noexcept { return !_Left; }

From the memory standard header.

By including <memory> in the module in which import nlohmann.json; is imported the issue is resolved.

I hope that helps. Let me know.

Cheers, Todd

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