Mask from bitfield in C++

Here's a little puzzle I couldn't find a good answer for:

Given a struct with bitfields, such as

struct A {
    unsigned foo:13;
    unsigned bar:19;
};

Is there a (portable) way in C++ to get the correct mask for one of the bitfields, preferably as a compile-time constant function or template?

Something like this:

constinit unsigned mask = getmask<A::bar>();  // mask should be 0xFFFFE000

In theory, at runtime, I could crudely do:

unsigned getmask_bar() {
    union AA {
        unsigned mask;
        A fields;
    } aa{};
    aa.fields.bar -= 1;
    return aa.mask;
}

That could even be wrapped in a macro (yuck!) to make it "generic".

But I guess you can readily see the various deficiencies of this method.

Is there a nicer, generic C++ way of doing it? Or even a not-so-nice way? Is there something useful coming up for the next C++ standard(s)? Reflection?