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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/99251] [11 Regression] inconsistent -Wnonnull warning
 behaviour with dynamic_cast
Date: Wed, 24 Feb 2021 18:14:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99251

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Depends on|                            |74762

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
The reason why there's no warning for cl3::g() is because the result of the
cast is not dereferenced in the same expression (the -Wnonnull warning is
issued for the call, and the call is in the next statement).

The reason why there's no warning for the parenthesized cast in cl3::h() is=
 due
to pr74762: the C++ front end sets the no-warning bit on the parenthesized
expression.  The warning sees this IL:

  cl2::h (((struct cl3 *) this)->p !=3D 0B ? (struct cl2 *) __dynamic_cast
(this->p, &_ZTI3cl1, &_ZTI3cl2, 0) : 0B)

where both the COND_EXPR (?:) and the NE_EXPR (!=3D) have the no-warning bi=
t set
and the warning code uses the first bit to suppress it.

The reason why there is a warning for cl3::i() is because the no-warning bi=
t is
set only on the NE_EXPR and not on the COND_EXPR as above, and the warning =
code
only tests the latter.

Finally, the reason why the warning is not issued for a similar static_cast
(where the argument has to be checked for equality to null in order for the
result to stay null) is because of the fix for pr96003 that set the no-warn=
ing
bit even on the COND_EXPR but didn't make the corresponding change in
ifnonnull() in cp/rtti.c.  What a mess.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D74762
[Bug 74762] [8/9/10/11 Regression] missing uninitialized warning (C++,
parenthesized expr, TREE_NO_WARNING)=