json: Fail to build when including json.hpp as a system include

What is the issue you have?

When including json.hpp if the include directory is marked as a system include (-isystem rather than -I) it will fail to build

I have to turn on CMAKE_NO_SYSTEM_FROM_IMPORTED and force the include path to be used as -I instead of -isystem. -isystem is used whenever nlohmann is provided as part of an imported CMake target. This happened to me due to using nlohmann from hunter with LLVM for Windows.

Please describe the steps to reproduce the issue. Can you provide a small but working code example?

Sample CMake script

project(test)
cmake_minimum_required(VERSION 3.14)

add_executable(test main.cpp)
target_include_directories(test SYSTEM PRIVATE "<path/to/nlohmann/json>")

Simple C++

#include <nlohmann/json.hpp>

int main()
{
// Don't even have to call anything
return 0;
}

What is the expected behavior?

Program compiles successfully

And what is the actual behavior instead?

A ton of errors along the line of error: expected '>'enable_if_t<is_detected<mapped_type_t, ConstructibleObjectType>::value and

Which compiler and operating system are you using? Is it a supported compiler?

Clang 9.0.0 but the same behavior occurs with 8.0.0/8.0.1

This is with LLVM for Windows and happens with the ClangCL toolset for VS2019 and the Ninja generator in CMake 3.15+

Did you use a released version of the library or the version from the develop branch?

Version 3.6.1 and 3.7.0 release versions have the same problem

If you experience a compilation error: can you compile and run the unit tests?

Nope

About this issue

Original URL
State: closed
Created 5 years ago
Comments: 55 (25 by maintainers)

Commits related to this issue

:white_check_mark: add test for target_include_directories #1818 — committed to nlohmann/json by nlohmann 4 years ago

Most upvoted comments

Oki I tested from the develop branch, everything is green, great!

tawmoto on Jul 17, 2020

Unfortunately I don’t have the clang setup on my PC (I am using linux), it’s on a remote PC build machine with special rights. I hope today or tomorrow I can replicate it on my PC and try to replicate. 👍

tawmoto on Jun 4, 2020

The easiest way to fix this would just be to remove all instances of the alternate identifiers.

I’m really hesitant to making this change as these operators are valid C++ and we even include the <ciso646> header which should convince noncompliant compilers about this…

nlohmann on Dec 16, 2019

@Rogiel Could you provide a MWE?

Sure, here it is: https://github.com/Rogiel/nlohmann-json-pch-bug

If you comment out the hack at the end of CMakeLists.txt (that defines and, or and not to their respective operators) everything works fine but that is hacky and will 100% break on any non-toy example.

I have only managed to reproduce this with Windows clang++.exe (NOT clang-cl.exe).

Rogiel on Nov 28, 2019

Hey! Sorry for the late reply on this. I’ve just whipped up a MWE cmake project. Not sure how to do this sort of include pattern on godbolt. test.zip

If you build with main.cpp including “bad.h” build will fail. If you include “good.h” instead it will work.

Here’s bad.h:

#pragma once

#include <type_traits>
#include <iostream>
#include <ciso646>

template<bool B, typename T = void>
using enable_if_t = typename std::enable_if<B, T>::type;

template<typename T, typename = enable_if_t<std::is_pod<T>::value and std::is_trivial<T>::value>>
bool square(T param) {
    return param and param;
}

And good.h

#pragma once

#include <type_traits>
#include <iostream>

template<bool B, typename T = void>
using enable_if_t = typename std::enable_if<B, T>::type;

template<typename T, typename = enable_if_t<std::is_pod<T>::value && std::is_trivial<T>::value>>
bool square(T param) {
    return param && param;
}

Edit: Just to clarify with LLVM for Windows and Ninja installed I configure with: cmake -GNinja -Bbuild and build with cmake --build build

This only fails with clang as MSVC does not have a “system” include concept

I haven’t tested this on Linux or macos yet but I can if that would be helpful

Honeybunch on Nov 12, 2019