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From: "teodor_spaeren at riseup dot net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/114367] New: std::vector<bool> constexpr
 initialization doesn't start lifetime of array members
Date: Sat, 16 Mar 2024 21:50:44 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114367

            Bug ID: 114367
           Summary: std::vector<bool> constexpr initialization doesn't
                    start lifetime of array members
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: teodor_spaeren at riseup dot net
  Target Milestone: ---

#include<vector>
consteval int foo(){
    std::vector<bool>seen(26);
    return 0;
}
int main(){
    return foo();
}

This is the code. If you compile this with g++, it compiles fine, but with
clang it gives a warning about object lifetimes.

lel.cpp:7:12: error: call to consteval function 'foo' is not a constant
expression
    return foo();
           ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../include/c++/13=
.2.1/bits/stl_bvector.h:649:15:
note: assignment to object outside its lifetime is not allowed in a constant
expression
            __p[__i] =3D 0ul;
                     ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../include/c++/13=
.2.1/bits/stl_bvector.h:1317:31:
note: in call to '&seen->_M_allocate(1)'
            _Bit_pointer __q =3D this->_M_allocate(__n);
                                     ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../include/c++/13=
.2.1/bits/stl_bvector.h:768:2:
note: in call to '&seen->_M_initialize(26)'
        _M_initialize(__n);
        ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../include/c++/13=
.2.1/bits/stl_bvector.h:755:9:
note: in call to 'vector(26, false, allocator_type())'
      : vector(__n, false, __a)
        ^
lel.cpp:3:22: note: in call to 'vector(26, allocator_type())'
    std::vector<bool>seen(26);
                     ^
lel.cpp:7:12: note: in call to 'foo()'
    return foo();
           ^
1 error generated.

When compiling with=20

https://github.com/gcc-mirror/gcc/blob/releases/gcc-13/libstdc%2B%2B-v3/inc=
lude/bits/stl_bvector.h#L676-L685

In the code it seems it never constructs the array, but starts accessing the
members directly. I don't know if this is affected by
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0593r6.html or no=
t.
I've submitted a bug report to clang also.=