JSON build fails for C++ cmake

[alexandraBara]

What is the issue you have?

I suspect recent changes in the library broke building JSON for C++. It worked up until a couple days ago.
I have added a code snipped of the fail below.

Please describe the steps to reproduce the issue.

Can you provide a small but working code example?

``` Downloading https://github.com/nlohmann/json/archive/HEAD.tar.gz

-- [download 100% complete]

-- The CXX compiler identification is Clang 13.0.0

-- Check for working CXX compiler: /opt/rocm/llvm/bin/clang++

-- Check for working CXX compiler: /opt/rocm/llvm/bin/clang++ -- works

-- Detecting CXX compiler ABI info

-- Detecting CXX compiler ABI info - done

-- Detecting CXX compile features

-- Detecting CXX compile features - done

-- Using the single-header code from /tmp/cget-16-46-27-gKybD-1/download/json-6d8d043adde8c95589f6e65e1fe228cb1f0d8419/single_include/

-- Found Git: /usr/bin/git (found version "2.17.1")

-- Operating system: Linux-4.15.0-163-generic; Distributor ID: Ubuntu; Description: Ubuntu 18.04.6 LTS; Release: 18.04; Codename: bionic; Linux 9e0027017371 4.15.0-163-generic #171-Ubuntu SMP Fri Nov 5 11:55:11 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

-- Compiler: AMD clang version 13.0.0 (https://github.com/RadeonOpenCompute/llvm-project roc-4.5.0 21422 e2489b0d7ede612d6586c61728db321047833ed8); Target: x86_64-unknown-linux-gnu; Thread model: posix; InstalledDir: /opt/rocm/llvm/bin

-- Configuring done

-- Generating done

CMake Warning:

Manually-specified variables were not used by the project:

CGET_CMAKE_ORIGINAL_SOURCE_FILE

-- Build files have been written to: /tmp/cget-16-46-27-gKybD-1/build

Scanning dependencies of target doctest_main

[ 0%] Building CXX object test/CMakeFiles/doctest_main.dir/src/unit.cpp.o

[ 0%] Built target doctest_main

Scanning dependencies of target test-wstring

[ 1%] Building CXX object test/CMakeFiles/test-wstring.dir/src/unit-wstring.cpp.o

Scanning dependencies of target test-ordered_map

Scanning dependencies of target test-regression2

Scanning dependencies of target test-unicode1

Scanning dependencies of target test-element_access2

Scanning dependencies of target test-allocator

Scanning dependencies of target test-serialization

Scanning dependencies of target test-items

Scanning dependencies of target test-diagnostics

Scanning dependencies of target test-readme

Scanning dependencies of target test-assert_macro

Scanning dependencies of target test-deserialization

Scanning dependencies of target test-disabled_exceptions

Scanning dependencies of target test-msgpack

Scanning dependencies of target test-algorithms

Scanning dependencies of target test-hash

Scanning dependencies of target test-alt-string

Scanning dependencies of target test-element_access1

Scanning dependencies of target test-testsuites

Scanning dependencies of target test-class_iterator

Scanning dependencies of target test-cbor

Scanning dependencies of target test-constructor1

Scanning dependencies of target test-constructor2

Scanning dependencies of target test-regression2_cpp17

Scanning dependencies of target test-conversions

Scanning dependencies of target test-conversions_cpp17

Scanning dependencies of target test-udt_macro

Scanning dependencies of target test-convenience

Scanning dependencies of target test-modifiers

Scanning dependencies of target test-unicode5

Scanning dependencies of target test-pointer_access

Scanning dependencies of target test-ordered_json

Scanning dependencies of target test-noexcept

Scanning dependencies of target test-to_chars

Scanning dependencies of target test-class_const_iterator

Scanning dependencies of target test-items_cpp17

Scanning dependencies of target test-reference_access

Scanning dependencies of target test-comparison

Scanning dependencies of target test-ubjson

Scanning dependencies of target test-bson

Scanning dependencies of target test-byte_container_with_subtype

Scanning dependencies of target test-capacity

Scanning dependencies of target test-class_lexer

Scanning dependencies of target test-regression1

Scanning dependencies of target test-udt

Scanning dependencies of target test-json_patch

Scanning dependencies of target test-user_defined_input

Scanning dependencies of target test-iterators2

Scanning dependencies of target test-unicode3

[ 4%] Building CXX object test/CMakeFiles/test-regression2.dir/src/unit-regression2.cpp.o

[ 4%] Building CXX object test/CMakeFiles/test-ordered_map.dir/src/unit-ordered_map.cpp.o

Scanning dependencies of target test-class_parser

Scanning dependencies of target test-regression1_cpp17

[ 5%] Building CXX object test/CMakeFiles/test-allocator.dir/src/unit-allocator.cpp.o

[ 6%] Building CXX object test/CMakeFiles/test-element_access2.dir/src/unit-element_access2.cpp.o

Scanning dependencies of target test-unicode2

[ 9%] Building CXX object test/CMakeFiles/test-deserialization.dir/src/unit-deserialization.cpp.o

[ 9%] Building CXX object test/CMakeFiles/test-class_iterator.dir/src/unit-class_iterator.cpp.o

[ 7%] Building CXX object test/CMakeFiles/test-readme.dir/src/unit-readme.cpp.o

[ 9%] Building CXX object test/CMakeFiles/test-serialization.dir/src/unit-serialization.cpp.o

[ 10%] Building CXX object test/CMakeFiles/test-constructor1.dir/src/unit-constructor1.cpp.o

[ 10%] Building CXX object test/CMakeFiles/test-regression2_cpp17.dir/src/unit-regression2.cpp.o

[ 11%] Building CXX object test/CMakeFiles/test-alt-string.dir/src/unit-alt-string.cpp.o

Scanning dependencies of target test-meta

[ 12%] Building CXX object test/CMakeFiles/test-algorithms.dir/src/unit-algorithms.cpp.o

[ 13%] Building CXX object test/CMakeFiles/test-hash.dir/src/unit-hash.cpp.o

[ 16%] Building CXX object test/CMakeFiles/test-element_access1.dir/src/unit-element_access1.cpp.o

[ 16%] Building CXX object test/CMakeFiles/test-items.dir/src/unit-items.cpp.o

[ 16%] Building CXX object test/CMakeFiles/test-disabled_exceptions.dir/src/unit-disabled_exceptions.cpp.o

Scanning dependencies of target test-iterators1

Scanning dependencies of target test-inspection

Scanning dependencies of target test-concepts

[ 17%] Building CXX object test/CMakeFiles/test-conversions.dir/src/unit-conversions.cpp.o

[ 17%] Building CXX object test/CMakeFiles/test-constructor2.dir/src/unit-constructor2.cpp.o

Scanning dependencies of target test-large_json

Scanning dependencies of target test-merge_patch

Scanning dependencies of target test-json_pointer

Scanning dependencies of target test-unicode4

[ 18%] Building CXX object test/CMakeFiles/test-diagnostics.dir/src/unit-diagnostics.cpp.o

[ 18%] Building CXX object test/CMakeFiles/test-conversions_cpp17.dir/src/unit-conversions.cpp.o

[ 19%] Building CXX object test/CMakeFiles/test-assert_macro.dir/src/unit-assert_macro.cpp.o

[ 20%] Building CXX object test/CMakeFiles/test-msgpack.dir/src/unit-msgpack.cpp.o

[ 20%] Building CXX object test/CMakeFiles/test-cbor.dir/src/unit-cbor.cpp.o

[ 21%] Building CXX object test/CMakeFiles/test-testsuites.dir/src/unit-testsuites.cpp.o

[ 22%] Building CXX object test/CMakeFiles/test-unicode1.dir/src/unit-unicode1.cpp.o

[ 22%] Building CXX object test/CMakeFiles/test-unicode5.dir/src/unit-unicode5.cpp.o

[ 23%] Building CXX object test/CMakeFiles/test-user_defined_input.dir/src/unit-user_defined_input.cpp.o

[ 23%] Building CXX object test/CMakeFiles/test-regression1.dir/src/unit-regression1.cpp.o

[ 23%] Building CXX object test/CMakeFiles/test-modifiers.dir/src/unit-modifiers.cpp.o

[ 25%] Building CXX object test/CMakeFiles/test-pointer_access.dir/src/unit-pointer_access.cpp.o

[ 24%] Building CXX object test/CMakeFiles/test-udt_macro.dir/src/unit-udt_macro.cpp.o

[ 26%] Building CXX object test/CMakeFiles/test-ordered_json.dir/src/unit-ordered_json.cpp.o

[ 27%] Building CXX object test/CMakeFiles/test-items_cpp17.dir/src/unit-items.cpp.o

[ 28%] Building CXX object test/CMakeFiles/test-regression1_cpp17.dir/src/unit-regression1.cpp.o

[ 29%] Building CXX object test/CMakeFiles/test-bson.dir/src/unit-bson.cpp.o

[ 30%] Building CXX object test/CMakeFiles/test-noexcept.dir/src/unit-noexcept.cpp.o

[ 31%] Building CXX object test/CMakeFiles/test-class_parser.dir/src/unit-class_parser.cpp.o

[ 32%] Building CXX object test/CMakeFiles/test-meta.dir/src/unit-meta.cpp.o

[ 32%] Building CXX object test/CMakeFiles/test-unicode2.dir/src/unit-unicode2.cpp.o

[ 33%] Building CXX object test/CMakeFiles/test-byte_container_with_subtype.dir/src/unit-byte_container_with_subtype.cpp.o

[ 34%] Building CXX object test/CMakeFiles/test-class_lexer.dir/src/unit-class_lexer.cpp.o

[ 34%] Building CXX object test/CMakeFiles/test-json_pointer.dir/src/unit-json_pointer.cpp.o

[ 35%] Building CXX object test/CMakeFiles/test-reference_access.dir/src/unit-reference_access.cpp.o

[ 36%] Building CXX object test/CMakeFiles/test-class_const_iterator.dir/src/unit-class_const_iterator.cpp.o

[ 41%] Building CXX object test/CMakeFiles/test-convenience.dir/src/unit-convenience.cpp.o

[ 42%] Building CXX object test/CMakeFiles/test-comparison.dir/src/unit-comparison.cpp.o

[ 41%] Building CXX object test/CMakeFiles/test-unicode3.dir/src/unit-unicode3.cpp.o

[ 41%] Building CXX object test/CMakeFiles/test-udt.dir/src/unit-udt.cpp.o

[ 41%] Building CXX object test/CMakeFiles/test-to_chars.dir/src/unit-to_chars.cpp.o

[ 42%] Building CXX object test/CMakeFiles/test-ubjson.dir/src/unit-ubjson.cpp.o

[ 43%] Building CXX object test/CMakeFiles/test-large_json.dir/src/unit-large_json.cpp.o

[ 43%] Building CXX object test/CMakeFiles/test-merge_patch.dir/src/unit-merge_patch.cpp.o

[ 44%] Building CXX object test/CMakeFiles/test-json_patch.dir/src/unit-json_patch.cpp.o

[ 45%] Building CXX object test/CMakeFiles/test-iterators1.dir/src/unit-iterators1.cpp.o

[ 46%] Building CXX object test/CMakeFiles/test-capacity.dir/src/unit-capacity.cpp.o

[ 47%] Building CXX object test/CMakeFiles/test-unicode4.dir/src/unit-unicode4.cpp.o

[ 48%] Building CXX object test/CMakeFiles/test-concepts.dir/src/unit-concepts.cpp.o

[ 50%] Building CXX object test/CMakeFiles/test-iterators2.dir/src/unit-iterators2.cpp.o

[ 50%] Building CXX object test/CMakeFiles/test-inspection.dir/src/unit-inspection.cpp.o

[ 50%] Linking CXX executable test-noexcept

[ 50%] Built target test-noexcept

[ 51%] Linking CXX executable test-byte_container_with_subtype

[ 52%] Linking CXX executable test-assert_macro

[ 53%] Linking CXX executable test-class_lexer

[ 53%] Built target test-byte_container_with_subtype

[ 54%] Linking CXX executable test-large_json

[ 54%] Built target test-assert_macro

[ 54%] Built target test-class_lexer

[ 54%] Built target test-large_json

[ 55%] Linking CXX executable test-to_chars

[ 56%] Linking CXX executable test-ordered_map

[ 56%] Built target test-to_chars

[ 57%] Linking CXX executable test-unicode4

[ 58%] Linking CXX executable test-wstring

[ 58%] Built target test-ordered_map

[ 58%] Built target test-unicode4

[ 58%] Built target test-wstring

[ 59%] Linking CXX executable test-concepts

[ 60%] Linking CXX executable test-convenience

[ 61%] Linking CXX executable test-disabled_exceptions

[ 62%] Linking CXX executable test-unicode2

[ 62%] Built target test-concepts

[ 62%] Built target test-disabled_exceptions

[ 62%] Built target test-convenience

[ 62%] Built target test-unicode2

[ 63%] Linking CXX executable test-unicode3

[ 63%] Linking CXX executable test-constructor2

[ 63%] Built target test-unicode3

[ 63%] Built target test-constructor2

[ 64%] Linking CXX executable test-capacity

[ 65%] Linking CXX executable test-unicode1

[ 65%] Built target test-capacity

[ 66%] Linking CXX executable test-unicode5

[ 66%] Built target test-unicode1

[ 66%] Built target test-unicode5

[ 67%] Linking CXX executable test-class_iterator

[ 67%] Built target test-class_iterator

[ 68%] Linking CXX executable test-hash

[ 69%] Linking CXX executable test-class_const_iterator

[ 70%] Linking CXX executable test-diagnostics

[ 70%] Built target test-hash

[ 71%] Linking CXX executable test-user_defined_input

[ 71%] Built target test-class_const_iterator

[ 71%] Built target test-diagnostics

[ 71%] Linking CXX executable test-allocator

[ 71%] Built target test-user_defined_input

[ 71%] Linking CXX executable test-items

[ 72%] Linking CXX executable test-merge_patch

[ 72%] Built target test-allocator

[ 73%] Linking CXX executable test-reference_access

[ 73%] Built target test-items

[ 74%] Linking CXX executable test-serialization

[ 74%] Built target test-merge_patch

[ 74%] Built target test-reference_access

[ 75%] Linking CXX executable test-meta

[ 75%] Built target test-serialization

[ 76%] Linking CXX executable test-algorithms

[ 76%] Built target test-meta

[ 76%] Built target test-algorithms

[ 77%] Linking CXX executable test-alt-string

[ 77%] Linking CXX executable test-testsuites

[ 77%] Built target test-alt-string

[ 78%] Linking CXX executable test-ordered_json

[ 78%] Built target test-testsuites

[ 78%] Linking CXX executable test-pointer_access

[ 78%] Built target test-ordered_json

[ 78%] Built target test-pointer_access

[ 79%] Linking CXX executable test-inspection

[ 79%] Linking CXX executable test-comparison

[ 80%] Linking CXX executable test-json_pointer

[ 80%] Built target test-inspection

[ 80%] Built target test-comparison

[ 81%] Linking CXX executable test-iterators1

[ 81%] Built target test-json_pointer

[ 81%] Built target test-iterators1

[ 81%] Linking CXX executable test-iterators2

[ 83%] Linking CXX executable test-items_cpp17

[ 83%] Linking CXX executable test-json_patch

[ 84%] Linking CXX executable test-element_access1

[ 85%] Linking CXX executable test-udt_macro

[ 86%] Linking CXX executable test-modifiers

[ 86%] Built target test-modifiers

[ 86%] Built target test-element_access1

[ 86%] Built target test-items_cpp17

[ 86%] Built target test-udt_macro

[ 86%] Built target test-iterators2

[ 86%] Built target test-json_patch

[ 87%] Linking CXX executable test-class_parser

[ 87%] Linking CXX executable test-bson

[ 87%] Linking CXX executable test-element_access2

[ 87%] Built target test-class_parser

[ 87%] Built target test-bson

[ 87%] Built target test-element_access2

[ 88%] Linking CXX executable test-readme

[ 88%] Built target test-readme

[ 89%] Linking CXX executable test-msgpack

[ 89%] Built target test-msgpack

[ 90%] Linking CXX executable test-ubjson

[ 90%] Built target test-ubjson

[ 90%] Linking CXX executable test-udt

[ 90%] Built target test-udt

[ 91%] Linking CXX executable test-cbor

[ 91%] Built target test-cbor

[ 92%] Linking CXX executable test-constructor1

[ 92%] Built target test-constructor1

[ 93%] Linking CXX executable test-regression2

[ 93%] Built target test-regression2

[ 94%] Linking CXX executable test-deserialization

[ 94%] Built target test-deserialization

[ 95%] Linking CXX executable test-conversions

[ 96%] Linking CXX executable test-regression2_cpp17

ld.lld: error: undefined symbol: std::experimental::filesystem::v1::__cxx11::path::_M_split_cmpts()

referenced by unit-regression2.cpp

          CMakeFiles/test-regression2_cpp17.dir/src/unit-regression2.cpp.o:(std::experimental::filesystem::v1::__cxx11::path::path<char [14], std::experimental::filesystem::v1::__cxx11::path>(char const (&) [14]))

referenced by unit-regression2.cpp

          CMakeFiles/test-regression2_cpp17.dir/src/unit-regression2.cpp.o:(std::experimental::filesystem::v1::__cxx11::path::path<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::experimental::filesystem::v1::__cxx11::path>(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&))

referenced by unit-regression2.cpp

          CMakeFiles/test-regression2_cpp17.dir/src/unit-regression2.cpp.o:(std::experimental::filesystem::v1::__cxx11::path::clear())

ld.lld: error: undefined symbol: std::experimental::filesystem::v1::__cxx11::path::compare(std::experimental::filesystem::v1::__cxx11::path const&) const

referenced by unit-regression2.cpp

          CMakeFiles/test-regression2_cpp17.dir/src/unit-regression2.cpp.o:(std::experimental::filesystem::v1::__cxx11::operator==(std::experimental::filesystem::v1::__cxx11::path const&, std::experimental::filesystem::v1::__cxx11::path const&))

clang-13: error: linker command failed with exit code 1 (use -v to see invocation)

test/CMakeFiles/test-regression2_cpp17.dir/build.make:85: recipe for target 'test/test-regression2_cpp17' failed

make[2]: *** [test/test-regression2_cpp17] Error 1

make[1]: *** [test/CMakeFiles/test-regression2_cpp17.dir/all] Error 2

make[1]: *** Waiting for unfinished jobs....

CMakeFiles/Makefile2:2065: recipe for target 'test/CMakeFiles/test-regression2_cpp17.dir/all' failed

[ 96%] Built target test-conversions

[ 97%] Linking CXX executable test-conversions_cpp17

[ 97%] Built target test-conversions_cpp17

[ 98%] Linking CXX executable test-regression1

[ 98%] Built target test-regression1

[100%] Linking CXX executable test-regression1_cpp17

[100%] Built target test-regression1_cpp17

Makefile:140: recipe for target 'all' failed

make: *** [all] Error 2

CMake Error at /usr/local/share/cmake/cmakeget/CMakeGet.cmake:142 (message):

Process failed: /usr/bin/cmake --build /tmp/cget-16-46-27-gKybD-1/build

--config Release -- -j 128

Call Stack (most recent call first):

/usr/local/share/cmake/cmakeget/CMakeGet.cmake:199 (cget_exec)

/usr/local/share/cmake/cmakeget/CMakeGet.cmake:401 (cget_install_dir)

/usr/local/share/cmake/cmakeget/CMakeGet.cmake:461 (cmake_get)

install_deps.cmake:90 (cmake_get_from)

</summary>
<!-- Please understand that we cannot analyze and debug large code bases. -->

#### What is the expected behavior?

<!-- Tell us what should happen -->

#### And what is the actual behavior instead?

<!-- Tell us what happens instead. -->

#### Which compiler and operating system are you using?

<!-- Include as many relevant details about the environment you experienced the bug in. -->
<!-- Make sure you use a supported compiler, see https://github.com/nlohmann/json#supported-compilers. -->

- Compiler: ___
- Operating system: ___

#### Which version of the library did you use?

<!-- Please add an `x` to the respective line. -->

- [ ] latest release version 3.10.5
- [ ] other release - please state the version: ___
- [ ] the `develop` branch

#### If you experience a compilation error: can you [compile and run the unit tests](https://github.com/nlohmann/json#execute-unit-tests)?

- [ ] yes
- [ ] no - please copy/paste the error message below

[nlohmann]

It seems that the compiler does not support std::filesystem, or we do not properly detect the support.

Just to make sure: can you please check the latest release 3.10.5?

[alexandraBara]

It seems that the compiler does not support std::filesystem, or we do not properly detect the support.

Just to make sure: can you please check the latest release 3.10.5?

We are using clang 13 and this has std::filesystem support since I believe version 9.
I specified version 3.10.5 and the same error persists.

[nlohmann]

We use Clang 13 in the CI without issues. 🤔

The error mention

std::experimental::filesystem::v1::__cxx11::path::_M_split_cmpts()

which makes no sense if the compiler has filesystem support. We use the following preprocessor checks:

        #if defined(__cpp_lib_filesystem)
            #define JSON_HAS_FILESYSTEM 1
        #elif defined(__cpp_lib_experimental_filesystem)
            #define JSON_HAS_EXPERIMENTAL_FILESYSTEM 1
        #elif !defined(__has_include)
            #define JSON_HAS_EXPERIMENTAL_FILESYSTEM 1
        #elif __has_include(<filesystem>)
            #define JSON_HAS_FILESYSTEM 1
        #elif __has_include(<experimental/filesystem>)
            #define JSON_HAS_EXPERIMENTAL_FILESYSTEM 1
        #endif

#ifndef JSON_HAS_EXPERIMENTAL_FILESYSTEM
    #define JSON_HAS_EXPERIMENTAL_FILESYSTEM 0
#endif

#ifndef JSON_HAS_FILESYSTEM
    #define JSON_HAS_FILESYSTEM 0
#endif

Can you please execute this with your compiler in C++17 mode and check which values are set for JSON_HAS_FILESYSTEM and JSON_HAS_EXPERIMENTAL_FILESYSTEM?

[NicolasIRAGNE]

Hello,

I do not think this error comes from json.
Your error mentions std::experimental::filesystem which as far as I know, was the pre-standard implementation. With the following code:

#include <experimental/filesystem>
#include <iostream>

int main(int ac, char **av)
{
    std::experimental::filesystem::path p(".");
    std::experimental::filesystem::path p2("..");

    std::cout << p.compare(p2) << std::endl;
}

I get the following errors:

➜  TestClang clang++-13 main.cpp -std=c++17
/usr/bin/ld: /tmp/main-13e873.o: in function `main':
main.cpp:(.text+0x42): undefined reference to `std::experimental::filesystem::v1::__cxx11::path::compare(std::experimental::filesystem::v1::__cxx11::path const&) const'
/usr/bin/ld: /tmp/main-13e873.o: in function `std::experimental::filesystem::v1::__cxx11::path::path<char [2], std::experimental::filesystem::v1::__cxx11::path>(char const (&) [2])':
main.cpp:(.text._ZNSt12experimental10filesystem2v17__cxx114pathC2IA2_cS3_EERKT_[_ZNSt12experimental10filesystem2v17__cxx114pathC2IA2_cS3_EERKT_]+0x55): undefined reference to `std::experimental::filesystem::v1::__cxx11::path::_M_split_cmpts()'
/usr/bin/ld: /tmp/main-13e873.o: in function `std::experimental::filesystem::v1::__cxx11::path::path<char [3], std::experimental::filesystem::v1::__cxx11::path>(char const (&) [3])':
main.cpp:(.text._ZNSt12experimental10filesystem2v17__cxx114pathC2IA3_cS3_EERKT_[_ZNSt12experimental10filesystem2v17__cxx114pathC2IA3_cS3_EERKT_]+0x55): undefined reference to `std::experimental::filesystem::v1::__cxx11::path::_M_split_cmpts()'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Replacing std::experimental::filesystem with std::filesystem compiles with no issue.
Do you happen to use experimental/filesystem in your code by any chance ?

Edit: I'm an idiot and didn't see that the code snippet that was posted is supposed to handle exactly what I said. Whoops. Sorry. It's still weird that it is trying to link with the experimental:: version however.

[nlohmann]

Can you please check the code from #3256 (comment) and post what is detected for your compiler?

[alexandraBara]

  #if defined(__cpp_lib_filesystem)
            #define JSON_HAS_FILESYSTEM 1
        #elif defined(__cpp_lib_experimental_filesystem)
            #define JSON_HAS_EXPERIMENTAL_FILESYSTEM 1
        #elif !defined(__has_include)
            #define JSON_HAS_EXPERIMENTAL_FILESYSTEM 1
        #elif __has_include(<filesystem>)
            #define JSON_HAS_FILESYSTEM 1
        #elif __has_include(<experimental/filesystem>)
            #define JSON_HAS_EXPERIMENTAL_FILESYSTEM 1
        #endif

I ran @NicolasIRAGNE code, here is the result:
JSON_HAS_EXPERIMENATL_FILESYSTEM is 1
JSON_HAS_FILESYSTEM is 0

clang version 13.0.0 (https://github.com/RadeonOpenCompute/llvm-project roc-4.3.0 21295 f2943f684437d2c1143a56e418d29fc6b3314072)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/rocm/llvm/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@MX32
Selected multilib: .;@m64
Found HIP installation: /opt/rocm, version 4.3.21300-5bbc51d8
"/opt/rocm-4.3.0/llvm/bin/clang-13" -cc1 -triple x86_64-unknown-linux-gnu -emit-obj -mrelax-all --mrelax-relocations -disable-free -disable-llvm-verifier -discard-value-names -main-file-name test.cpp -mrelocation-model static -mframe-pointer=all -fmath-errno -fno-rounding-math -mconstructor-aliases -munwind-tables -target-cpu x86-64 -tune-cpu generic -fno-split-dwarf-inlining -debugger-tuning=gdb -v -resource-dir /opt/rocm-4.3.0/llvm/lib/clang/13.0.0 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/x86_64-linux-gnu/c++/7.5.0 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/x86_64-linux-gnu/c++/7.5.0 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0/backward -internal-isystem /usr/local/include -internal-isystem /opt/rocm-4.3.0/llvm/lib/clang/13.0.0/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -Wall -std=c++17 -fdeprecated-macro -fdebug-compilation-dir=/home/fpadmin -ferror-limit 19 -fgnuc-version=4.2.1 -fcxx-exceptions -fexceptions -fcolor-diagnostics -faddrsig -o /tmp/test-f9b375.o -x c++ test.cpp
clang -cc1 version 13.0.0 based upon LLVM 13.0.0git default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/include"
ignoring duplicate directory "/usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/x86_64-linux-gnu/c++/7.5.0"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0
/usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/x86_64-linux-gnu/c++/7.5.0
/usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0/backward
/usr/local/include
/opt/rocm-4.3.0/llvm/lib/clang/13.0.0/include
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
test.cpp:28:9: warning: JSON_HAS_EXPERIMENATL_F: 1 [-W#pragma-messages]
#pragma message "JSON_HAS_EXPERIMENATL_F: " XSTR(JSON_HAS_EXPERIMENTAL_FILESYSTEM)
^
test.cpp:29:9: warning: JSON_HAS_FILESYSTEM: 0 [-W#pragma-messages]
#pragma message "JSON_HAS_FILESYSTEM: " XSTR(JSON_HAS_FILESYSTEM)
^
2 warnings generated.
"/opt/rocm/llvm/bin/ld.lld" -z relro --hash-style=gnu --eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o a.out /usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../x86_64-linux-gnu/crt1.o /usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/7.5.0/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/7.5.0 -L/usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../x86_64-linux-gnu -L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../.. -L/opt/rocm-4.3.0/llvm/bin/../lib -L/lib -L/usr/lib /tmp/test-f9b375.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/x86_64-linux-gnu/7.5.0/crtend.o /usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../x86_64-linux-gnu/crtn.o
ld.lld: error: undefined symbol: std::experimental::filesystem::v1::__cxx11::path::compare(std::experimental::filesystem::v1::__cxx11::path const&) const

referenced by test.cpp
/tmp/test-f9b375.o:(main)

ld.lld: error: undefined symbol: std::experimental::filesystem::v1::__cxx11::path::_M_split_cmpts()

referenced by test.cpp
/tmp/test-f9b375.o:(std::experimental::filesystem::v1::__cxx11::path::path<char [2], std::experimental::filesystem::v1::__cxx11::path>(char const (&) [2]))
referenced by test.cpp
/tmp/test-f9b375.o:(std::experimental::filesystem::v1::__cxx11::path::path<char [3], std::experimental::filesystem::v1::__cxx11::path>(char const (&) [3]))
clang-13: error: linker command failed with exit code 1 (use -v to see invocation)

[gregmarr]

Looks like it's somehow using the GCC 7.5.0 standard library instead of libc++.

#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0
/usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/x86_64-linux-gnu/c++/7.5.0
/usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0/backward

[alexandraBara]

Looks like it's somehow using the GCC 7.5.0 standard library instead of libc++.

#include "..." search starts here: #include <...> search starts here: /usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0 /usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/x86_64-linux-gnu/c++/7.5.0 /usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0/backward

I think that is OK. I was able to run this code by adding the '-lstdc++fs'

[nlohmann]

So the library correctly detected that the used libc++ only supports <experimental/filesystem>, but the CMake files for the test suite compilation did not link stdc++fs, because it expected the compiler did not need it. Right?

[alexandraBara]

So the library correctly detected that the used libc++ only supports <experimental/filesystem>, but the CMake files for the test suite compilation did not link stdc++fs, because it expected the compiler did not need it. Right?

That is correct, the library correctly detected the experimental/filesystem :). Please feel free to close this issue and thank you for helping out.

[nlohmann]

Thanks!

[snorrikris]

Unrelated maybe... But I had a strange issue when building release (x64) build in VS2022 v17.1.2 using c++20 (I'm playing with modules). Same project builds in Debug x64.
This is the error:
error LNK2001: unresolved external symbol "bool __cdecl std::filesystem::operator==(class std::filesystem::_Path_iterator<template-type-parameter-1'> const &,class std::filesystem::_Path_iterator<template-type-parameter-1'> const &)" (??8filesystem@std@@YA_NAEBV?$_Path_iterator@$RBAAB@@01@0@Z)

I finally solved this by adding in json.hpp at line 54:
#define JSON_HAS_CPP_14

FYI - I have tried countless things before posting here. Including removing any code using std::filesystem. Nlohmann json is the only library using std::filesystem in my project, that's why I suspected a problem in the json code. I just though I should mention this in case others are having the same problem. But it would be nice to find a solution of course.