[Bug libstdc++/111258] New: std::string cannot to be moved in constant evaluated expression

[Bug libstdc++/111258] New: std::string cannot to be moved in constant evaluated expression

[kisses07_rupee at icloud dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111258

            Bug ID: 111258
           Summary: std::string cannot to be moved in constant evaluated
                    expression
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kisses07_rupee at icloud dot com
  Target Milestone: ---

c++20 std::string's destructor fails after move:

#include <string>

using namespace std::literals;

constexpr auto foo(auto a) {
    auto b = std::move(a);
    return b[1];
}

int main(int,char**) {
    static_assert( foo("hello"s) == 'e' );
    return 0;
}

fails with:
<source>: In function 'int main(int, char**)':
<source>:11:34: error: non-constant condition for static assertion
   11 |     static_assert( foo("hello"s) == 'e' );
      |                    ~~~~~~~~~~~~~~^~~~~~
<source>:11:23:   in 'constexpr' expansion of
'foo<std::__cxx11::basic_string<char>
>(std::literals::string_literals::operator""s(const char*, std::size_t)(5))'
<source>:11:42:   in 'constexpr' expansion of
'std::__cxx11::basic_string<char>((* &
std::move<__cxx11::basic_string<char>&>(a)))'
<source>:11:34: error: accessing 'std::__cxx11::basic_string<char>::<unnamed
union>::_M_allocated_capacity' member instead of initialized
'std::__cxx11::basic_string<char>::<unnamed union>::_M_local_buf' member in
constant expression
Compiler returned: 1

online demo: https://godbolt.org/z/Tz7MK58f1

it seems it's affecting on bug 103924

[Bug libstdc++/111258] std::string cannot to be moved in constant evaluated expression

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111258

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note clang rejects (libstdc++ version) for a different reason:
```
<source>:12:20: error: static assertion expression is not an integral constant
expression
   12 |     static_assert( foo("hello"s) == 'e' );
      |                    ^~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/14.0.0/../../../../include/c++/14.0.0/bits/basic_string.h:626:2:
note: undefined function '_M_construct<const char *>' cannot be used in a
constant expression
  626 |         _M_construct(__s, __s + __n, std::forward_iterator_tag());
      |         ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/14.0.0/../../../../include/c++/14.0.0/bits/basic_string.h:4686:14:
note: in call to 'basic_string(&"hello"[0], 5, std::allocator<char>())'
 4686 |     { return basic_string<char>{__str, __len}; }
      |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:12:24: note: in call to 'operator""s(&"hello"[0], 5)'
   12 |     static_assert( foo("hello"s) == 'e' );
      |                        ^~~~~~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/14.0.0/../../../../include/c++/14.0.0/bits/basic_string.h:336:9:
note: declared here
  336 |         _M_construct(_FwdIterator __beg, _FwdIterator __end,
      |         ^
```

clang accepts LLVM's libc++ version though.

[Bug libstdc++/111258] std::string cannot to be moved in constant evaluated expression

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111258

Jiang An <de34 at live dot cn> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |de34 at live dot cn

--- Comment #2 from Jiang An <de34 at live dot cn> ---
Perhaps this is a compiler bug and there's nothing wrong in the move
constructor.

GCC 13 also incorrectly rejects the following program
(https://godbolt.org/z/Yn98KMKv5)
```
#include <string>

constexpr bool foo11(std::string s)
{
    s.push_back('\0');
    return true;
}

int main()
{
    static_assert(foo11(std::string{}));
}

```

with similar message:

```
<source>: In function 'int main()':
<source>:12:24: error: non-constant condition for static assertion
   12 |     static_assert(foo11(std::string{}));
      |                   ~~~~~^~~~~~~~~~~~~~~
<source>:12:24:   in 'constexpr' expansion of
'foo11(std::__cxx11::basic_string<char>())'
<source>:5:16:   in 'constexpr' expansion of
's.std::__cxx11::basic_string<char>::push_back(0)'
/opt/compiler-explorer/gcc-13.2.0/include/c++/13.2.0/bits/basic_string.h:1552:33:
  in 'constexpr' expansion of
'((std::__cxx11::basic_string<char>*)this)->std::__cxx11::basic_string<char>::capacity()'
<source>:12:24: error: accessing 'std::__cxx11::basic_string<char>::<unnamed
union>::_M_allocated_capacity' member instead of initialized
'std::__cxx11::basic_string<char>::<unnamed union>::_M_local_buf' member in
constant expression
Compiler returned: 1
```

It seems that gcc currently miscompiles passing-by-value of
non-trivially-copyable classes in constant evaluation.

[Bug libstdc++/111258] std::string cannot to be moved in constant evaluated expression

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111258

--- Comment #3 from Jiang An <de34 at live dot cn> ---
I've reduced the example and filed Bug 111284.

[Bug libstdc++/111258] std::string cannot to be moved in constant evaluated expression

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111258

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Many of these testcases (from here and the related PRs) are now accepted by GCC
14 after r14-10134.