is_output_streamable doesn't compiled on Visual Studio if T==std::nullptr_t

[wisk]

Hi,

It seems is_output_streamable<std::nullptr_t> cause an error on Visual Studio because std::cout << nullptr; is ambiguous according to cl compiler.

error C2593: 'operator <<' is ambiguous

To mitigate this issue, I just wrote a specialized template for this function:

template<>
struct is_output_streamable<std::nullptr_t>
{
  static const bool value = false;
}

But I don't know if this is the best fix, I found another solution here.