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

[daniel.kruegler at googlemail dot com]

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

--- Comment #7 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
(In reply to Matthieu Brucher from comment #6)
> I never said that the test alone should be banned. Please read the original
> message first.

I had done so before I replied. And it seems that I'm not the only one who has
problems understanding your request.

> I said that if(!this) in the context of a method gives "unexpected" behavior
> (according to the standard and the difference in behavior of GCC between
> debug and optimized mode) and thus should give an error or at least a
> warning, and this is easy to catch.

I don't understand this. Please provide an example.

> At least make it a warning, like clang does when it detects that a path is
> always true or false, some kind of way for the user to _know_ that they
> messed up. 

I already suggested the same thing: The only reason to request a warning in
this case is because the test result has only one outcome. But this is
absolutely unrelated to undefined behaviour unless you clarify what precisely
your mean.

> And yes, there is something fundamentally wrong with comparing this to
> nullptr according to the C++ standard and the contract GCC has in its
> optimizer. 

You claim this repeatedly but I'm missing a proof for the "fundamentally wrong"
part. Except if you consider tautologies as "fundamentally wrong".

> Or give a reason why it is valid and where the standard says it
> supercedes the undefined behavior and the thread discussion on why GCC added
> the optimization constraint.
> And in that case, you should probably remove the optimization as well to be
> consistent.

This part is unclear without a specific example that you are talking about, see
above.