Add C++17 copies of the test binaries

[nlohmann]

This PR makes sure C++17 is used additionally where the test suite is using C++17 features.

Fixes #3090.

[coveralls]

Coverage remained the same at 100.0% when pulling eba77cd on issue3090 into 825d323 on develop.

[nlohmann]

For some reason, the test with Clang9 on Ubuntu is currently failing with a segmentation fault. It would be great if anyone had an idea.

[theodelrieu]

I wonder if we should bother supporting non-conforming C++17 compilers?

Or at least we could rely on __has_include to check for filesystem then experimental/filesystem.

[gregmarr]

Or at least we could rely on __has_include to check for filesystem then experimental/filesystem.

That's what this PR is doing, after the feature test macros, which are supposed to be better indicators.

[theodelrieu]

@nlohmann Looks like it still chooses the CompatibleArrayType overload on that Ubuntu-clang9. I don't have such a setup near me unfortunately.

[nlohmann]

You can run that configuration in Docker with

docker run -it ghcr.io/nlohmann/json-ci:v1.0.0 /bin/bash
git pull https://github.com/nlohmann/json.git
cd json
git checkout issue3090
cmake -S . -B build -DJSON_CI=On
cmake --build build --target ci_test_compiler_clang++-9

Running it individually, I get:

===============================================================================
/root/json/test/src/unit-regression2.cpp:302:
TEST CASE:  regression tests 2
  issue #3070 - Version 3.10.3 breaks backward-compatibility with 3.10.2 

/root/json/test/src/unit-regression2.cpp:808: SUCCESS: CHECK( j_path == text_path ) is correct!
  values: CHECK( "/tmp/text.txt" == "/tmp/text.txt" )

/root/json/test/src/unit-regression2.cpp:810: SUCCESS: CHECK_THROWS_WITH_AS( nlohmann::detail::std_fs::path(json(1)), "[json.exception.type_error.302] type must be string, but is number", json::type_error ) threw as expected!

/root/json/test/src/unit-regression2.cpp:302: FATAL ERROR: test case CRASHED: SIGSEGV - Segmentation violation signal
```�

When running in gdb, I get this stacktrace:

Program received signal SIGSEGV, Segmentation fault.
0x00007f0ff5adecb0 in std::filesystem::__cxx11::path::_List::_Impl_deleter::operator()(std::filesystem::__cxx11::path::_List::_Impl*) const ()
from /lib/x86_64-linux-gnu/libstdc++.so.6
(gdb) bt
#0 0x00007f0ff5adecb0 in std::filesystem::__cxx11::path::_List::_Impl_deleter::operator()(std::filesystem::__cxx11::path::_List::_Impl*) const ()
from /lib/x86_64-linux-gnu/libstdc++.so.6
#1 0x0000000000687a1d in std::unique_ptr<std::filesystem::__cxx11::path::_List::_Impl, std::filesystem::__cxx11::path::_List::_Impl_deleter>::unique_ptr
(this=0x7ffe27733d60) at /usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/unique_ptr.h:361
#2 0x0000000000687955 in std::filesystem::__cxx11::path::_List::_List (this=0x7ffe27733d60)
at /usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/fs_path.h:649
#3 0x00000000005ab2b6 in std::filesystem::__cxx11::path::~path (this=0x7ffe27733d40)
at /usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/fs_path.h:310
#4 0x000000000059dd24 in _DOCTEST_ANON_FUNC_2 () at /root/json/test/src/unit-regression2.cpp:811
#5 0x0000000000572797 in doctest::Context::run (this=0x7ffe27737b38) at /root/json/test/thirdparty/doctest/doctest.h:6486
#6 0x0000000000573816 in main (argc=2, argv=0x7ffe27737c48) at /root/json/test/thirdparty/doctest/doctest.h:6571

[YarikTH]

#if !defined(JSON_HAS_FILESYSTEM) && !defined(JSON_HAS_EXPERIMENTAL_FILESYSTEM)
    #ifdef JSON_HAS_CPP_17
        #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

        // std::filesystem does not work on MinGW GCC 8: https://sourceforge.net/p/mingw-w64/bugs/737/
        #if defined(__MINGW32__) && defined(__GNUC__) && __GNUC__ == 8
            #undef JSON_HAS_FILESYSTEM
            #undef JSON_HAS_EXPERIMENTAL_FILESYSTEM
        #endif

        // no filesystem support before GCC 8: https://en.cppreference.com/w/cpp/compiler_support
        #if defined(__GNUC__) && !defined(__clang__) && __GNUC__ < 8
            #undef JSON_HAS_FILESYSTEM
            #undef JSON_HAS_EXPERIMENTAL_FILESYSTEM
        #endif

        // no filesystem support before Clang 7: https://en.cppreference.com/w/cpp/compiler_support
        #if defined(__clang_major__) && __clang_major__ < 7
            #undef JSON_HAS_FILESYSTEM
            #undef JSON_HAS_EXPERIMENTAL_FILESYSTEM
        #endif

        // no filesystem support before MSVC 19.14: https://en.cppreference.com/w/cpp/compiler_support
        #if defined(_MSC_VER) && _MSC_VER < 1940
            #undef JSON_HAS_FILESYSTEM
            #undef JSON_HAS_EXPERIMENTAL_FILESYSTEM
        #endif

        // no filesystem support before iOS 13
        #if defined(__IPHONE_OS_VERSION_MIN_REQUIRED) && __IPHONE_OS_VERSION_MIN_REQUIRED < 130000
            #undef JSON_HAS_FILESYSTEM
            #undef JSON_HAS_EXPERIMENTAL_FILESYSTEM
        #endif

        // no filesystem support before macOS Catalina
        #if defined(__MAC_OS_X_VERSION_MIN_REQUIRED) && __MAC_OS_X_VERSION_MIN_REQUIRED < 101500
            #undef JSON_HAS_FILESYSTEM
            #undef JSON_HAS_EXPERIMENTAL_FILESYSTEM
        #endif
    #endif
#endif

#ifndef JSON_HAS_EXPERIMENTAL_FILESYSTEM
    #define JSON_HAS_EXPERIMENTAL_FILESYSTEM 0
#endif

#ifndef JSON_HAS_FILESYSTEM
    #define JSON_HAS_FILESYSTEM 0
#endif

Why do you combine has_include checks with version range checks? As a result, for example on GCC7, there is a more or less working experimental filesystem and has_include should detect this, but in the version range check you just erase results for all has_include checks just because some outdated table from the internet says so. Looks strange.

It doesn't bother my projects, I don't pass filesystem path via json and I have defines to override the behaviour, so for me it is still fine.

Also duplicating block worth its own header in details.

#if JSON_HAS_EXPERIMENTAL_FILESYSTEM
#include <experimental/filesystem>
namespace nlohmann::detail
{
namespace std_fs = std::experimental::filesystem;
} // namespace nlohmann::detail
#elif JSON_HAS_FILESYSTEM
#include <filesystem>
namespace nlohmann::detail
{
namespace std_fs = std::filesystem;
} // namespace nlohmann::detail
#endif

[nlohmann]

@YarikTH Thanks for the hints. The current state was a lot of fiddling, and I am currently happy with the way it is. That said, if you have any proposals how to improve, feel free to open a PR.