[Bug c++/81928] if(!this) optimization leads to possible errors without warnings

["matthieu.brucher at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81928

--- Comment #19 from Matthieu Brucher <matthieu.brucher at gmail dot com> ---
That was my original comment... Thanks for quoting me...
The issue is that the warning doesn't tell me what you told me, that "this" is
never equal to nullptr in that context. This is what the warning should be (and
as such, yes, it is a bug).

This change in behavior (this is always different from nullptr) was introduced
in gcc 6. Any version before would still check that the condition is true, so
GCC behaves better now, handling better the case than before, but it still
needs to educate people that what they are doing is wrong.