[Bug c++/81928] New: 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

            Bug ID: 81928
           Summary: if(!this) optimization leads to possible errors
                    without warnings
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: matthieu.brucher at gmail dot com
  Target Milestone: ---

The constructs if(!this) in a method raises a warning about comparison with
NULL, as it should.
We agree that this is an undefined behavior at its worst. The issue is that the
behavior of this condition is not the same between debug and optimized where
the call is considered at compile time.
Even if there is an existing warning on this undefined behavior, this should be
an error (or a warning no matter what) as there is a different behavior between
debug and optimized (even if they both make sense).