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

[redi at gcc dot gnu.org]

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

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Matthieu Brucher from comment #2)
> Comparing this to anything from Null to nullptr should be an error, not be
> silently passed.

No, it can't be rejected, because it's valid code. Just like "if (!true)" is
valid code. Just because it's pointless and maybe a mistake or a bug doesn't
mean we can reject it.