Constexpr friend functions in MSVC

I'm trying to define constexpr friend operator in template. I got a compiler error when trying to instantiate this operator in non-constexpr context. If I define the very same operator as a template class member of as a free template function, it works fine.

template <typename T>
struct A
{
    T Value;

    // error
    friend constexpr bool operator== (const A& l, const A& r)
    {
        return l.Value == r.Value;
    }

    // ok
    constexpr bool operator!= (const A& r) const
    {
        return Value != r.Value;
    }
};

// ok
template <typename T> constexpr bool
operator< (const A<T>& l, const A<T>& r)
{
    return l.Value < r.Value;
}

#include <string>
int main ()
{
    A<std::string> s;
    bool ret = (s < s, s == s, s != s);
}

The error I've got is

<source>(7): error C3615: constexpr function 'operator ==' cannot result in a constant expression

<source>(9): note: failure was caused by call of undefined function or one not declared 'constexpr'

<source>(9): note: see usage of 'std::operator =='

<source>(8): note: while compiling class template member function 'bool operator ==(const A<std::string> &,const A<std::string> &)'

<source>(29): note: see reference to class template instantiation 'A<std::string>' being compiled

Godbolt link

Is this 'friend' discrimination is a requirement of the standard or a compiler bug?