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* [Bug c++/109671] New: Spurious dangling reference warning in GCC 13
@ 2023-04-28 17:48 lopresti at gmail dot com
2023-04-28 17:51 ` [Bug c++/109671] " pinskia at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: lopresti at gmail dot com @ 2023-04-28 17:48 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109671
Bug ID: 109671
Summary: Spurious dangling reference warning in GCC 13
Product: gcc
Version: 13.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: lopresti at gmail dot com
Target Milestone: ---
#include <string>
struct Foo;
extern Foo &get_foo_by_name(const std::string &name);
const Foo &bug(bool x)
{
const Foo &f = get_foo_by_name(x ? "x" : "y");
return f;
}
---
Compile with "-O2 -Wall" to get the incorrect warning:
<source>: In function 'const Foo& bug(bool)':
<source>:9:14: warning: possibly dangling reference to a temporary
[-Wdangling-reference]
9 | const Foo &f = get_foo_by_name(x ? "x" : "y");
| ^
<source>:9:33: note: the temporary was destroyed at the end of the full
expression 'get_foo_by_name(std::__cxx11::basic_string<char>(((const char*)(x ?
"x" : "y")), std::allocator<char>()))'
9 | const Foo &f = get_foo_by_name(x ? "x" : "y");
| ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
--
Godbolt: https://godbolt.org/z/cn4W7ohGb
The code is fine. (And no other compiler warns about this, including earlier
GCC versions.)
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/109671] Spurious dangling reference warning in GCC 13
2023-04-28 17:48 [Bug c++/109671] New: Spurious dangling reference warning in GCC 13 lopresti at gmail dot com
@ 2023-04-28 17:51 ` pinskia at gcc dot gnu.org
2023-04-28 19:31 ` lopresti at gmail dot com
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-04-28 17:51 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109671
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There is no way for GCC to know that get_foo_by_name does not store the
argument into what is returned so it warns about this case ...
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/109671] Spurious dangling reference warning in GCC 13
2023-04-28 17:48 [Bug c++/109671] New: Spurious dangling reference warning in GCC 13 lopresti at gmail dot com
2023-04-28 17:51 ` [Bug c++/109671] " pinskia at gcc dot gnu.org
@ 2023-04-28 19:31 ` lopresti at gmail dot com
2023-05-01 1:15 ` lopresti at gmail dot com
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: lopresti at gmail dot com @ 2023-04-28 19:31 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109671
--- Comment #2 from Patrick J. LoPresti <lopresti at gmail dot com> ---
Um... OK...
So I have to "correct" my code like so:
const Foo &bug(bool x)
{
const std::string s = (x ? "x" : "y");
const Foo &f = get_foo_by_name(s);
return f;
}
But if get_foo_by_name() has the problem GCC is worried about, this does not
even fix it.
This warning seems silly to me. "I cannot tell if you are managing lifetimes
properly, so I will just recommend you do not use temporaries"?
Const references have always bound to temporaries. This warning seems to be
discouraging that (?)
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/109671] Spurious dangling reference warning in GCC 13
2023-04-28 17:48 [Bug c++/109671] New: Spurious dangling reference warning in GCC 13 lopresti at gmail dot com
2023-04-28 17:51 ` [Bug c++/109671] " pinskia at gcc dot gnu.org
2023-04-28 19:31 ` lopresti at gmail dot com
@ 2023-05-01 1:15 ` lopresti at gmail dot com
2023-05-02 19:50 ` cvs-commit at gcc dot gnu.org
2024-01-19 17:35 ` mpolacek at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: lopresti at gmail dot com @ 2023-05-01 1:15 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109671
--- Comment #3 from Patrick J. LoPresti <lopresti at gmail dot com> ---
(In reply to Andrew Pinski from comment #1)
> There is no way for GCC to know that get_foo_by_name does not store the
> argument into what is returned so it warns about this case ...
To summarize:
GCC is warning because it does not and cannot know whether I am managing object
lifetimes correctly.
The only way to silence the warning is to modify my code to make it uglier,
into a form where GCC still does not and cannot know whether I am managing
lifetimes correctly.
So is this a valid bug report, or not?
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/109671] Spurious dangling reference warning in GCC 13
2023-04-28 17:48 [Bug c++/109671] New: Spurious dangling reference warning in GCC 13 lopresti at gmail dot com
` (2 preceding siblings ...)
2023-05-01 1:15 ` lopresti at gmail dot com
@ 2023-05-02 19:50 ` cvs-commit at gcc dot gnu.org
2024-01-19 17:35 ` mpolacek at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2023-05-02 19:50 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109671
--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-13 branch has been updated by Marek Polacek
<mpolacek@gcc.gnu.org>:
https://gcc.gnu.org/g:6b927b1297e66e26e62e722bf15c921dcbbd25b9
commit r13-7276-g6b927b1297e66e26e62e722bf15c921dcbbd25b9
Author: Marek Polacek <polacek@redhat.com>
Date: Tue May 2 15:48:40 2023 -0400
c++: Move -Wdangling-reference to -Wextra [PR109642]
Sadly, -Wdangling-reference generates false positives for std::span-like
user classes, and it seems imprudent to attempt to improve the heuristic
in GCC 13. Let's move the warning to -Wextra, that will hopefully
reduce the number of false positives the users have been seeing with 13.
I'm leaving the warning in -Wall in 14 where I think I can write code
to detect std::span-like classes.
PR c++/109642
PR c++/109640
PR c++/109671
gcc/c-family/ChangeLog:
* c.opt (Wdangling-reference): Move from -Wall to -Wextra.
gcc/ChangeLog:
* doc/invoke.texi: Document that -Wdangling-reference is
enabled by -Wextra.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/109671] Spurious dangling reference warning in GCC 13
2023-04-28 17:48 [Bug c++/109671] New: Spurious dangling reference warning in GCC 13 lopresti at gmail dot com
` (3 preceding siblings ...)
2023-05-02 19:50 ` cvs-commit at gcc dot gnu.org
@ 2024-01-19 17:35 ` mpolacek at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: mpolacek at gcc dot gnu.org @ 2024-01-19 17:35 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109671
Marek Polacek <mpolacek at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |DUPLICATE
CC| |mpolacek at gcc dot gnu.org
Status|UNCONFIRMED |RESOLVED
--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to Patrick J. LoPresti from comment #3)
> (In reply to Andrew Pinski from comment #1)
> > There is no way for GCC to know that get_foo_by_name does not store the
> > argument into what is returned so it warns about this case ...
>
> To summarize:
>
> GCC is warning because it does not and cannot know whether I am managing
> object lifetimes correctly.
>
> The only way to silence the warning is to modify my code to make it uglier,
> into a form where GCC still does not and cannot know whether I am managing
> lifetimes correctly.
>
> So is this a valid bug report, or not?
It's valid but also I don't think I can tweak the heuristic not to warn here.
There ought to be a simpler way to suppress the warning, see bug 109642.
*** This bug has been marked as a duplicate of bug 109642 ***
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2024-01-19 17:35 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2023-04-28 17:48 [Bug c++/109671] New: Spurious dangling reference warning in GCC 13 lopresti at gmail dot com
2023-04-28 17:51 ` [Bug c++/109671] " pinskia at gcc dot gnu.org
2023-04-28 19:31 ` lopresti at gmail dot com
2023-05-01 1:15 ` lopresti at gmail dot com
2023-05-02 19:50 ` cvs-commit at gcc dot gnu.org
2024-01-19 17:35 ` mpolacek at gcc dot gnu.org
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