[Bug libstdc++/56551] A faster implementation of std::function

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56551

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
As I just wrote on the duplicate bug:

I've looked into this and I don't think it can be done without an ABI break.
Objects constructed in old code compiled by an older GCC would still have a
null pointer, so code compiled by a new GCC can't assume that the pointer is
never null.