[Bug c++/61867] gcc can't detect obviously false test

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61867

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to David Binderman from comment #3)
> I think that all that needs to happen is a warning is produced
> where either the detection or reduction takes place.

There is no single place, it's a result of constant propagation and a whole
range of optimisations cooperating. Those optimisations are a good thing, you
don't want to spit out a warning every time the compiler decides it can remove
part of the code, you'd end up with either hundreds of warnings for correct
code or disabling all optimisations.

> As ever, users are free to ignore warnings. egrep -v is
> useful, I find.

egrep is useless for ignoring warnings. It might help on the command line, but
not if you run the compiler from an editor or IDE, or with -Werror etc.

Just because you don't mind ignoring warnings doesn't mean it is appropriate
for GCC to start spitting out poor quality warnings.