Inject namespace experimental to std

Is it bad or good parctice to inject namespace std::experimental into std like following?

namespace std
{
namespace experimental
{
}
using namespace experimental;    
}

#include <experimental/optional>

int main()
{
    std::optional< int > o;
    return 0;
}

Or even in more modern form:

#if __has_include(<optional>)
# include <optional>
#elif __has_include(<experimental/optional>)
# include <experimental/optional>
namespace std
{
using namespace experimental;    
}
#else
#error !
#endif

int main()
{
    std::optional< int > o;
    return 0;
}

The intention to introduce std::experimental "sub-namespace" is clear because std::experimental currently contains a plenty of new libraries. I think it is very likely all them will migrate into namespace std without any substantial changes and user code written currently can rely upon this (am I totally wrong?). Otherwise all this code should be refactored to change from std::experimental:: to std:: in the future. It is not big deal, but there may be reasons not to do so.

The question is about both production code and not-too-serious code.