[Bug libstdc++/113294] New: constexpr error from accessing inactive union member in basic_string after move assignment

[Bug libstdc++/113294] New: constexpr error from accessing inactive union member in basic_string after move assignment

[pkeir at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113294

            Bug ID: 113294
           Summary: constexpr error from accessing inactive union member
                    in basic_string after move assignment
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pkeir at outlook dot com
  Target Milestone: ---

The C++20 program below fails to compile. A call to `basic_string::clear()` in
the move assignment operator leads to an access of inactive union member
`_M_local_buf`.

#include <string>

constexpr bool string_test1()
{
  {
    std::string str1; //  = "1"; // also a problem when small (less than 16)
    std::string str2 = "1234567890123456"; // 16 chars
    str1 = std::move(str2);
  }

  return true;
}

static_assert(string_test1());

Changing `__str._M_data(__str._M_local_buf);` to
`__str._M_data(__str._M_use_local_data());` in basic_string.h seems to fix it,
though I've not had time to test it fully. With this solution, the function
below also remains functional:

constexpr bool string_test2()                                                   
{                                                                               
  {                                                                             
    std::string str = "1234567890123456";  // 16 chars                          
    str = std::string{"1234567890123456"}; // rvalue assignment                 
  }                                                                             

  return true;                                                                  
}

[Bug libstdc++/113294] constexpr error from accessing inactive union member in basic_string after move assignment

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113294

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note this might be a front-end issue ...

[Bug libstdc++/113294] constexpr error from accessing inactive union member in basic_string after move assignment

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113294

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-01-23
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
The front end started rejecting the testcase (with libstdc++ 12, 13 and 14
headers) after the active union member change tracking improvements r14-4771. 
Clang also rejects the testcase with libstdc++ 13 headers, which points to this
being a library bug unlike the other related PRs about constexpr std::string. 
Your proposed fix looks good to me.

[Bug libstdc++/113294] constexpr error from accessing inactive union member in basic_string after move assignment

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113294

--- Comment #3 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:065dddc6e07a917c57c7955db13b1fe77abbcabc

commit r14-8943-g065dddc6e07a917c57c7955db13b1fe77abbcabc
Author: Paul Keir <paul.keir@uws.ac.uk>
Date:   Mon Feb 12 18:15:49 2024 +0000

    libstdc++: Fix constexpr basic_string union member [PR113294]

    A call to `basic_string::clear()` in the std::string move assignment
    operator leads to a constexpr error from an access of inactive union
    member `_M_local_buf` in the added test (`test_move()`). Changing
    `__str._M_local_buf` to `__str._M_use_local_data()` in
    `operator=(basic_string&& __str)` fixes this.

            PR libstdc++/113294

    libstdc++-v3/ChangeLog:

            * include/bits/basic_string.h (basic_string::operator=): Use
            _M_use_local_data() instead of _M_local_buf on the moved-from
            string.
            * testsuite/21_strings/basic_string/modifiers/constexpr.cc
            (test_move): New test.

    Signed-off-by: Paul Keir <paul.keir@uws.ac.uk>
    Reviewed-by: Patrick Palka <ppalka@redhat.com>
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

[Bug libstdc++/113294] constexpr error from accessing inactive union member in basic_string after move assignment

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113294

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-13 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:ebe00c9d3a0436dec5c354a62d98e444d763ff95

commit r13-8335-gebe00c9d3a0436dec5c354a62d98e444d763ff95
Author: Paul Keir <paul.keir@uws.ac.uk>
Date:   Mon Feb 12 18:15:49 2024 +0000

    libstdc++: Fix constexpr basic_string union member [PR113294]

    A call to `basic_string::clear()` in the std::string move assignment
    operator leads to a constexpr error from an access of inactive union
    member `_M_local_buf` in the added test (`test_move()`). Changing
    `__str._M_local_buf` to `__str._M_use_local_data()` in
    `operator=(basic_string&& __str)` fixes this.

            PR libstdc++/113294

    libstdc++-v3/ChangeLog:

            * include/bits/basic_string.h (basic_string::operator=): Use
            _M_use_local_data() instead of _M_local_buf on the moved-from
            string.
            * testsuite/21_strings/basic_string/modifiers/constexpr.cc
            (test_move): New test.

    Signed-off-by: Paul Keir <paul.keir@uws.ac.uk>
    Reviewed-by: Patrick Palka <ppalka@redhat.com>
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
    (cherry picked from commit 065dddc6e07a917c57c7955db13b1fe77abbcabc)

[Bug libstdc++/113294] constexpr error from accessing inactive union member in basic_string after move assignment

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113294

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
   Target Milestone|---                         |13.3
             Status|NEW                         |RESOLVED

--- Comment #5 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 13.3/14 thanks for the report and patch!