The local variable which is used in return statement doesn't convert to r-value implicitly to match the conversion operator

In the example snippet code below, the local variable which is used in return statement doesn't convert to r-value implicitly to match the conversion operator. However for move constructor it works.

I want to know whether it is a standard behavior or a bug. And if it is a standard behavior, what is the reason?

I tested it in Microsoft Visual Studio 2019 (Version 16.8.3) in 'permissive-' mode and it produced a compiler error. But in 'permissive' mode, it was OK.

#include <string>

class X
{
    std::string m_str;
public:
    X() = default;
    X(X&& that)
    {
        m_str = std::move(that.m_str);
    }
    operator std::string() &&
    {
        return std::move(m_str);
    }
};

X f()
{
    X x;
    return x;
}

std::string g()
{
    X x;
    return x; // Conformance mode: Yes (/permissive-) ==> error C2440: 'return': cannot convert from 'X' to 'std::basic_string<char,std::char_traits<char>,std::allocator<char>>'
    //return std::move(x); // OK
    // return X{}; // OK
}

int main()
{
    f();
    g();
    return 0;
}