[Bug c++/109640] New: Spurious Wdangling-reference for argument to temporary lambda cast to rvalue reference

[Bug c++/109640] New: Spurious Wdangling-reference for argument to temporary lambda cast to rvalue reference

[ed at catmur dot uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109640

            Bug ID: 109640
           Summary: Spurious Wdangling-reference for argument to temporary
                    lambda cast to rvalue reference
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ed at catmur dot uk
  Target Milestone: ---

bool b() {
    int a;
    int&& i = [](int& r) -> int&& { return static_cast<int&&>(r); }(a);
    auto const l = [](int& r) -> int&& { return static_cast<int&&>(r); };
    int&& j = l(a);
    return &i == &j;
}

<source>: In function 'bool b()':
<source>:3:11: warning: possibly dangling reference to a temporary
[-Wdangling-reference]
    3 |     int&& i = [](int& r) -> int&& { return static_cast<int&&>(r); }(a);
      |           ^
<source>:3:68: note: the temporary was destroyed at the end of the full
expression '<lambda closure
object>b()::<lambda(int&)>().b()::<lambda(int&)>(a)'
    3 |     int&& i = [](int& r) -> int&& { return static_cast<int&&>(r); }(a);
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~

The heuristic appears to be confused by the lambda itself being a temporary,
even though it is captureless.

[Bug c++/109640] Spurious Wdangling-reference for argument to temporary lambda cast to rvalue reference

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109640

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Reduced testcase:
```
struct f{};
int &&ffa(const f&, int &r) { return static_cast<int&&>(r); }
void b() {
  int a;
  int&& i = ffa(f{}, a);
  i = 1;
}
```

Should see f{} is an empty struct which is exactly the same as the lambda here.

[Bug c++/109640] Spurious Wdangling-reference for argument to temporary lambda cast to rvalue reference

[ed at catmur dot uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109640

--- Comment #2 from Ed Catmur <ed at catmur dot uk> ---
Ah, so this is Bug 108165? That's a shame, we use (temporary) lambdas
extensively so I think we'd have to disable the warning entirely.

[Bug c++/109640] Spurious Wdangling-reference for argument to temporary lambda cast to rvalue reference

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109640

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
We could probably disable the warning when the temporary is of empty class
type?

[Bug c++/109640] Spurious Wdangling-reference for argument to temporary lambda cast to rvalue reference

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109640

--- 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.

[Bug c++/109640] Spurious Wdangling-reference for argument to temporary lambda cast to rvalue reference

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109640

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
   Last reconfirmed|                            |2024-01-18
     Ever confirmed|0                           |1

[Bug c++/109640] Spurious Wdangling-reference for argument to temporary lambda cast to rvalue reference

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109640

--- Comment #5 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:9010fdba68063beccfbab0aa9ec8739f232ca2f4

commit r14-8375-g9010fdba68063beccfbab0aa9ec8739f232ca2f4
Author: Marek Polacek <polacek@redhat.com>
Date:   Fri Jan 19 13:59:41 2024 -0500

    c++: -Wdangling-reference and lambda false warning [PR109640]

    -Wdangling-reference checks if a function receives a temporary as its
    argument, and only warns if any of the arguments was a temporary.  But
    we should not warn when the temporary represents a lambda or we generate
    false positives as in the attached testcases.

            PR c++/113256
            PR c++/111607
            PR c++/109640

    gcc/cp/ChangeLog:

            * call.cc (do_warn_dangling_reference): Don't warn if the temporary
            is of lambda type.

    gcc/testsuite/ChangeLog:

            * g++.dg/warn/Wdangling-reference14.C: New test.
            * g++.dg/warn/Wdangling-reference15.C: New test.
            * g++.dg/warn/Wdangling-reference16.C: New test.

[Bug c++/109640] Spurious Wdangling-reference for argument to temporary lambda cast to rvalue reference

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109640

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Should be fixed.

[Bug c++/109640] Spurious Wdangling-reference for argument to temporary lambda cast to rvalue reference

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109640

--- Comment #7 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:f2061b2a9641c2228d4e2d86f19532ad7e93d627

commit r14-8636-gf2061b2a9641c2228d4e2d86f19532ad7e93d627
Author: Marek Polacek <polacek@redhat.com>
Date:   Thu Jan 25 12:08:14 2024 -0500

    c++: avoid -Wdangling-reference for std::span-like classes [PR110358]

    Real-world experience shows that -Wdangling-reference triggers for
    user-defined std::span-like classes a lot.  We can easily avoid that
    by considering classes like

        template<typename T>
        struct Span {
          T* data_;
          std::size len_;
        };

    to be std::span-like, and not warning for them.  Unlike the previous
    patch, this one considers a non-union class template that has a pointer
    data member and a trivial destructor as std::span-like.

            PR c++/110358
            PR c++/109640

    gcc/cp/ChangeLog:

            * call.cc (reference_like_class_p): Don't warn for std::span-like
            classes.

    gcc/ChangeLog:

            * doc/invoke.texi: Update -Wdangling-reference description.

    gcc/testsuite/ChangeLog:

            * g++.dg/warn/Wdangling-reference18.C: New test.
            * g++.dg/warn/Wdangling-reference19.C: New test.
            * g++.dg/warn/Wdangling-reference20.C: New test.