c++17 `filesystem` is not a namespace-name

Question

I am wondering why in the following code, the namespace filesystem is not found:

g++ -std=c++17 main.cpp -lstdc++

// #include <filesystem>   <- error, so changed to the following:
#include <experimental/filesystem>

namespace fs = std::filesystem;

int main()
{
    return 0;
}

error:

main.cpp:3:21: error: ‘filesystem’ is not a namespace-name
 namespace fs = std::filesystem;
                     ^
main.cpp:3:31: error: expected namespace-name before ‘;’ token
 namespace fs = std::filesystem;

gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.5)

Answer 1

GCC 5.4.0 was released in June of 2016; over a year before the C++17 standard was adopted. It and its version of libstdc++ have very limited C++17 support. You can see when GCC added C++17 language features here and when libstdc++ added C++17 standard library features here.

At the time of GCC 5.4's release, the filesystem library was not yet implemented in the std::filesystem namespace. It, along with any other <experimental/...> headers that are included in that version, are in the std::experimental namespace.