Is it possible to use std::char_traits in constant expressions before GCC 8?

According to the GCC 8 Release Note, it is now possible to use std::char_traits<char> and std::char_traits<wchar_t> in constant expressions:

• Improved experimental support for C++17, including the following features:
  • std::char_traits and std::char_traits<wchar_t> are usable in constant expressions.

What I understand here is you can’t use std::char_traits<char> in constant expressions before GCC 8.

However, the following code compiles well with GCC 7.5:

#include <string_view>

int main() {
    constexpr std::string_view sv{""};
    constexpr std::size_t size = std::char_traits<char>::length(sv.data());

    return 0;
}

So, is it possible to use std::char_traits in constant expressions before GCC 8? Or am I missing something?