[Bug c++/114709] New: Incorrect handling of inactive union member access via pointer to member in constant evaluated context

[Bug c++/114709] New: Incorrect handling of inactive union member access via pointer to member in constant evaluated context

[420 at zerberste dot es]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114709

            Bug ID: 114709
           Summary: Incorrect handling of inactive union member access via
                    pointer to member in constant evaluated context
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: 420 at zerberste dot es
  Target Milestone: ---

According to https://standards.pydong.org/c++/class.union#general-note-1 an
inactive union member may be used to inspect the initial sequence common with
the active union member, given both are struct types. Furthermore
https://standards.pydong.org/c++/class.mem#general-26 tells us the behavior of
reading a non-static member m of such a common initial sequence is _as if_ the
corresponding member were nominated. Since it is not expression-equivalent, we
are still accessing the wrong member as far as the abstract machine is
concerned. Hence https://standards.pydong.org/c++/expr.const#5.10 still
applies.

GCC implements this correctly for the simple case. To reuse the example from
[class.mem]/26 consider this code fragment:
```cpp
struct T1 { int a, b; };
struct T2 { int c; double d; };
union U { T1 t1; T2 t2; };

consteval int f() {
  U u = { { 1, 2 } }; // active member is t1
  return u.t2.c;      // access through t2
}

static constexpr auto foo = f();
```
https://godbolt.org/z/x14n47T45

GCC correctly errors with the diagnostic 
> accessing 'U::t2' member instead of initialized 'U::t1' member in constant expression


However, consider the following code fragment:
```cpp
struct T1 { int a, b; };
struct T2 { int c; double d; };
union U { T1 t1; T2 t2; };

consteval int f() {
  U u = { { 1, 2 } };  // active member is t1
  return u.t2.*&T2::c; // access through t2
}

static constexpr auto foo = f();
```
https://godbolt.org/z/dK95jEPjq

GCC 12 up until trunk accept this code. GCC 11 and Clang don't.

[Bug c++/114709] Incorrect handling of inactive union member access via pointer to member in constant evaluated context

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114709

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |accepts-invalid
   Last reconfirmed|                            |2024-04-13
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug c++/114709] [12/13/14 Regression] Incorrect handling of inactive union member access via pointer to member in constant evaluated context

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114709

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.4
      Known to work|                            |11.4.0
      Known to fail|                            |12.1.0
            Summary|Incorrect handling of       |[12/13/14 Regression]
                   |inactive union member       |Incorrect handling of
                   |access via pointer to       |inactive union member
                   |member in constant          |access via pointer to
                   |evaluated context           |member in constant
                   |                            |evaluated context
           Keywords|                            |needs-bisection

[Bug c++/114709] [12/13/14 Regression] Incorrect handling of inactive union member access via pointer to member in constant evaluated context

[law at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114709

Jeffrey A. Law <law at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2
                 CC|                            |law at gcc dot gnu.org

[Bug c++/114709] [12/13/14 Regression] Incorrect handling of inactive union member access via pointer to member in constant evaluated context since r12-6425

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114709

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[12/13/14 Regression]       |[12/13/14 Regression]
                   |Incorrect handling of       |Incorrect handling of
                   |inactive union member       |inactive union member
                   |access via pointer to       |access via pointer to
                   |member in constant          |member in constant
                   |evaluated context           |evaluated context since
                   |                            |r12-6425
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |ppalka at gcc dot gnu.org
           Keywords|needs-bisection             |

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This is accepted since r12-6425-gab36b554bd90e8db279d13b133369118814f13fb

[Bug c++/114709] [12/13/14 Regression] Incorrect handling of inactive union member access via pointer to member in constant evaluated context since r12-6425

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114709

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

[Bug c++/114709] [12/13/14 Regression] Incorrect handling of inactive union member access via pointer to member in constant evaluated context since r12-6425

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114709

--- 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:0844170e9ef60a8b2f6fba6786672f30ce1c2749

commit r14-10110-g0844170e9ef60a8b2f6fba6786672f30ce1c2749
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Apr 24 17:49:56 2024 -0400

    c++: constexpr union member access folding [PR114709]

    The object/offset canonicalization performed in cxx_fold_indirect_ref
    is undesirable for union member accesses because it loses information
    about the member being accessed which we may later need to diagnose an
    inactive-member access.  So this patch restricts the canonicalization
    accordingly.

            PR c++/114709

    gcc/cp/ChangeLog:

            * constexpr.cc (cxx_fold_indirect_ref): Restrict object/offset
            canonicalization to RECORD_TYPE member accesses.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/constexpr-union8.C: New test.

    Reviewed-by: Jason Merrill <jason@redhat.com>

[Bug c++/114709] [12/13 Regression] Incorrect handling of inactive union member access via pointer to member in constant evaluated context since r12-6425

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114709

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[12/13/14 Regression]       |[12/13 Regression]
                   |Incorrect handling of       |Incorrect handling of
                   |inactive union member       |inactive union member
                   |access via pointer to       |access via pointer to
                   |member in constant          |member in constant
                   |evaluated context since     |evaluated context since
                   |r12-6425                    |r12-6425

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 14 so far.