libstdc++/9828: Regression: Segmentation fault in num_put::put

[Benjamin Kosnik]

>OK, but if nothing outside the library depends on its size, then
>adding members at the end should be OK.  Right?

I think this depends on usage. 

If a __locale_cache pointer or auto_ptr is embedded in some struct
(which is not going to be a base class), and some kind of accessor
function a la use_cache is used to get the data, then things should be
cool regardless of it being exported or not. The preference is still to
not export, mostly because it give implementors much more flexibility to
tweak in the future. Furthermore, the accessor functions can be added
and versioned appropriately. 

-benjamin