Regression test for #3070 is not being run and fails when enabled

[falbrechtskirchinger]

Working on some unit test improvements, I discovered at least two unit tests that are not being run. This issue is about

#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
    SECTION("issue #3070 - Version 3.10.3 breaks backward-compatibility with 3.10.2 ")
    {
        static_assert(false); // Added to confirm this test is never run
        nlohmann::detail::std_fs::path text_path("/tmp/text.txt");
        json j(text_path);

        const auto j_path = j.get<nlohmann::detail::std_fs::path>();
        CHECK(j_path == text_path);

        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);
    }
#endif

added in 6d31159.

nlohmann::detail::std_fs::path(json(1)) fails to compile, because the constructor call is ambiguous.

@theodelrieu @nlohmann Was/is this expected to work?

(The fact, that this test isn't being run will be fixed by an upcoming PR.)

[falbrechtskirchinger]

Clarification: If I remember correctly the ambiguity arises from std::fs::path accepting both std::string and std::fs::path (copy ctor, obviously) and basic_json<> can implicitly convert to both. I don't see a way to resolve this ambiguity.

@nlohmann Thoughts?

Again, this has apparently never worked as the test was never executed.

[nlohmann]

Thanks for clarifying! I'll think about this. Good to know that it never worked - this allows us to make changes without breaking existing implementations ;-)

[nlohmann]

I am confused - locally, the test is executed and succeeds... (Apple clang version 13.0.0)

[nlohmann]

So it seems the test is indeed not executed on Windows - maybe the filesystem detection is broken and all compilers are skipped. The test is executed on macOS and Linux for some compilers. However, not for all C++17 enabled compilers. 🤔