[Bug c++/114706] New: ICE - std::bit_cast in consteval function involving union

[Bug c++/114706] New: ICE - std::bit_cast in consteval function involving union

[cuzdav at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114706

            Bug ID: 114706
           Summary: ICE - std::bit_cast in consteval function involving
                    union
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cuzdav at gmail dot com
  Target Milestone: ---

ICE in a consteval function bit_casting an array of unions.  The code isn't
valid due to the union, but the compiler is not handling it properly (since
gcc11...trunk).  Reduced to the following program:

CODE

    #include <bit>
    union U { int u; };
    consteval void f() {
        U result[]{0, 0};   
        auto x = std::bit_cast<long>(result);
    }
    int main() { f(); }

LIVE:
https://godbolt.org/z/Y943bc5zK


OUTPUT

/opt/compiler-explorer/gcc-trunk-20240412/include/c++/14.0.1/bit:94:33:
internal compiler error: in native_encode_initializer, at fold-const.cc:8468
   94 |       return __builtin_bit_cast(_To, __from);
      |                                 ^~~
0x267826c internal_error(char const*, ...)
        ???:0
0xa58c63 fancy_abort(char const*, int, char const*)
        ???:0
0xfa0eaf native_encode_initializer(tree_node*, unsigned char*, int, int,
unsigned char*)
        ???:0
0xacf0f0 maybe_constant_value(tree_node*, tree_node*, mce_value)
        ???:0
0xd3a75f store_init_value(tree_node*, tree_node*, vec<tree_node*, va_gc,
vl_embed>**, int)
        ???:0
0xb474fe cp_finish_decl(tree_node*, tree_node*, bool, tree_node*, int,
cp_decomp*)
        ???:0
0xc5c81a c_parse_file()
        ???:0
0xdb1489 c_common_parse_file()
        ???:0
Please submit a full bug report, with preprocessed source (by using
-freport-bug).
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.

[Bug c++/114706] ICE - std::bit_cast in consteval function involving union

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114706

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2024-04-12
           Keywords|                            |ice-on-invalid-code

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

[Bug c++/114706] ICE - std::bit_cast in consteval function involving array of union

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114706

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|ICE - std::bit_cast in      |ICE - std::bit_cast in
                   |consteval function          |consteval function
                   |involving union             |involving array of union

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Looks like check_bit_cast_type does not check ARRAY_TYPE recusively ...

[Bug c++/114706] ICE - std::bit_cast in consteval function involving array of union

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114706

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

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

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 57947
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57947&action=edit
gcc14-pr114706.patch

Untested obvious patch.

[Bug c++/114706] ICE - std::bit_cast in consteval function involving array of union

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114706

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:79ff53453e88e40c4f2ecf6f7f7409afc41b46fc

commit r14-9987-g79ff53453e88e40c4f2ecf6f7f7409afc41b46fc
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Apr 16 09:39:19 2024 +0200

    c++: Handle ARRAY_TYPE in check_bit_cast_type [PR114706]

    https://eel.is/c++draft/bit.cast#3 says that std::bit_cast isn't constexpr
    if To, From and the types of all subobjects have certain properties which
the
    check_bit_cast_type checks (such as it isn't a pointer, reference, union,
    member pointer, volatile).  The function doesn't cp_walk_tree though, so
    I've missed one important case, for ARRAY_TYPEs we need to recurse on the
    element type.  I think we don't need to handle VECTOR_TYPEs/COMPLEX_TYPEs,
    because those will not have a pointer/reference/union/member pointer in
    the element type and if the element type is volatile, I think the whole
    derived type is volatile as well.

    2024-04-16  Jakub Jelinek  <jakub@redhat.com>

            PR c++/114706
            * constexpr.cc (check_bit_cast_type): Handle ARRAY_TYPE.

            * g++.dg/cpp2a/bit-cast17.C: New test.

[Bug c++/114706] ICE - std::bit_cast in consteval function involving array of union

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114706

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

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

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed.