How do I reserve space on the stack for a non-default constructible?

I would basically write the following piece of code. I understand why it can't compile.

A instance; // A is a non-default-constructable type and therefore can't be allocated like this

if (something)
{
    instance = A("foo"); // use a constructor X
}
else
{
    instance = A(42); // use *another* constructor Y
}

instance.do_something();

Is there a way to achieve this behaviour without involving heap-allocation?