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From: "flast at flast dot jp" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107532] [13 Regression] -Werror=dangling-reference false
 positives in libcamera-0.0.1
Date: Tue, 14 Mar 2023 23:09:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107532

--- Comment #24 from Kohei Takahashi <flast at flast dot jp> ---
(In reply to Marek Polacek from comment #23)
> (In reply to Kohei Takahashi from comment #21)
> > (In reply to Marek Polacek from comment #18)
> > > (In reply to Barnab=C3=A1s P=C5=91cze from comment #17)
> > > > The simple test case with std::span still triggers the warning:
> > > > https://gcc.godbolt.org/z/43cKxdqr3. I feel that without deeper code
> > > > analysis such a warning will generate too many false positives and =
people
> > > > will simply turn it off.
> > >=20
> > > There really haven't been that many, except this and one with range-b=
ased
> > > for loops.
> >=20
> > I think it warns many usage of zip_iterator idiom such as boost.iterato=
r and
> > P2321 style flat_map. Those uses tuple of references like std::tuple<T&=
...>
> > by dereferencing iterator, so that any algorithms may yield this warnin=
g.
>=20
> Ah, would you please have a testcase?  If that's the case and the warning
> can't be taught to recognize that pattern, then I think we need to move it
> to -Wextra.  Thanks.

In my flat map implementation, https://github.com/Flast/flat_map, the warni=
ng
is shown here
https://github.com/Flast/flat_map/blob/f7d547fd4dbde763c07eb8d35796248c4198=
9a66/flat_map/__flat_tree.hpp#LL435C42-L435C52
. You can reproduce it by following
```
flat_map$ mkdir build
flat_map$ cd build
flat_map/build$ cmake ..
flat_map/build$ make map_tie_test_17
```

`_key_extractor` is defined here,
https://github.com/Flast/flat_map/blob/f7d547fd4dbde763c07eb8d35796248c4198=
9a66/flat_map/flat_map.hpp#L89-L90,
and the iterator dereference is here,
https://github.com/Flast/flat_map/blob/f7d547fd4dbde763c07eb8d35796248c4198=
9a66/flat_map/tied_sequence.hpp#L67-L72.
Hence, reduced code is like https://wandbox.org/permlink/DloAyU3dQgydo7PS, =
or
https://wandbox.org/permlink/7fM4NDF8u1hiRMFC.=