[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

Daniel Krügler <daniel.kruegler at googlemail dot com> changed:

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--- Comment #5 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
(In reply to Matthieu Brucher from comment #4)
> I would agree if the debug and optimized versions had the same behavior.
> They do not. As such there should be a huge warning about the undefined
> behavior. 

The test alone is not undefined behaviour. This is like requesting that the
following code has undefined behaviour:

int main() 
{
  int i = 12;
  int* that = &i;
  if (!that) {}
}

> And no, it's not valid code, it's an undefined behavior, you can do whatever
> you want with it. Outputting an error is a better solution, even adding a
> warning is.  Silently optimizing an undefined behavior away is not.

This argumentation is wrong. There is nothing fundamentally wrong to compare
'this' against null. The only reason to request a warning is because this test
cannot evaluate to true in conforming programs.