[Bug c++/96630] New: dangling reference accepted in constexpr evaluation

[Bug c++/96630] New: dangling reference accepted in constexpr evaluation

[gccbugbjorn at fahller dot se]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96630

            Bug ID: 96630
           Summary: dangling reference accepted in constexpr evaluation
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gccbugbjorn at fahller dot se
  Target Milestone: ---

Created attachment 49065
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49065&action=edit
Example code

This program uses a reference to an out-of-scope local variable in constexpr
context.

struct ii
{
    int v;
    constexpr const int& get() const { return v;}
    constexpr ii& inc() { ++v; return *this;}
};

struct wrap
{
    ii i;
    constexpr const int& getnext() const { auto copy = i; return
copy.inc().get();}
};

template <int> struct S {};
S<wrap{{3}}.getnext()> s;

It is, however, accepted and compiles. If used in runtime evaluation, the
undefined behaviour causes wildly different results depending on optimization
level.

bf :-) confuciusornis /tmp> g++-10 --version
g++-10 (Ubuntu 10-20200411-0ubuntu1) 10.0.1 20200411 (experimental) [master
revision bb87d5cc77d:75961caccb7:f883c46b4877f637e0fa5025b4d6b5c9040ec566]
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

bf :-) confuciusornis /tmp> g++-10 -std=c++20 -c -Wall -Wextra -pedantic t.cpp
bf :-) confuciusornis /tmp> 


Godbolt link: https://godbolt.org/z/8q91dv

[Bug c++/96630] dangling reference accepted in constexpr evaluation

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96630

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2020-08-17
           Keywords|                            |accepts-invalid
             Status|UNCONFIRMED                 |NEW

[Bug c++/96630] dangling reference accepted in constexpr evaluation

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96630

--- Comment #1 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:9fdbd7d6fa5e0a76898dd66658934e3184111680

commit r14-2773-g9fdbd7d6fa5e0a76898dd66658934e3184111680
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Sun Jul 23 01:15:14 2023 +1000

    c++: Track lifetimes in constant evaluation [PR70331,PR96630,PR98675]

    This adds rudimentary lifetime tracking in C++ constexpr contexts,
    allowing the compiler to report errors with using values after their
    backing has gone out of scope. We don't yet handle other ways of
    accessing values outside their lifetime (e.g. following explicit
    destructor calls).

            PR c++/96630
            PR c++/98675
            PR c++/70331

    gcc/cp/ChangeLog:

            * constexpr.cc (constexpr_global_ctx::is_outside_lifetime): New
            function.
            (constexpr_global_ctx::get_value): Don't return expired values.
            (constexpr_global_ctx::get_value_ptr): Likewise.
            (constexpr_global_ctx::remove_value): Mark value outside
            lifetime.
            (outside_lifetime_error): New function.
            (cxx_eval_call_expression): No longer track save_exprs.
            (cxx_eval_loop_expr): Likewise.
            (cxx_eval_constant_expression): Add checks for outside lifetime
            values. Remove local variables at end of bind exprs, and
            temporaries after cleanup points.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/constexpr-lifetime1.C: New test.
            * g++.dg/cpp1y/constexpr-lifetime2.C: New test.
            * g++.dg/cpp1y/constexpr-lifetime3.C: New test.
            * g++.dg/cpp1y/constexpr-lifetime4.C: New test.
            * g++.dg/cpp1y/constexpr-lifetime5.C: New test.
            * g++.dg/cpp1y/constexpr-lifetime6.C: New test.

    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>