[Bug c++/78615] error: cannot decrement a pointer to incomplete type

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78615

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I don't think this is valid code and here is why:
struct QListNode is incomplete.
You use an instantiation of QList<int> via class B which means all methods of
QList<int> are instantiated.