Why does holds_alternative build without std namespace

Question

Why does holds_alternative build without std namespace? I have tested this with godbolt.org using different compiles and it builds.

#include <variant>
int main()
{
    std::variant<int, double> foo = 1;
    bool wuff = holds_alternative<int>(foo);
    return 0;
}

Tested example with https://godbolt.org/z/1YoMq1seY with different compilers