[Bug libstdc++/103295] New: constexpr std::string does not work for clang

[Bug libstdc++/103295] New: constexpr std::string does not work for clang

[unlvsur at live dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

            Bug ID: 103295
           Summary: constexpr std::string does not work for clang
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: unlvsur at live dot com
  Target Milestone: ---

#include<string>
#include<cassert>

constexpr bool foo()
{
        std::string str2{"abcwe"};
        return str2.size()==5;
}

static_assert(foo());

int main()
{
        assert(foo());
}

[Bug libstdc++/103295] constexpr std::string does not work for clang

[unlvsur at live dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

cqwrteur <unlvsur at live dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |unlvsur at live dot com

--- Comment #1 from cqwrteur <unlvsur at live dot com> ---
Created attachment 51821
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51821&action=edit
clang error message

[Bug libstdc++/103295] constexpr std::string does not work for clang

[unlvsur at live dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #2 from cqwrteur <unlvsur at live dot com> ---
*** Bug 103294 has been marked as a duplicate of this bug. ***

[Bug libstdc++/103295] constexpr std::string does not work for clang

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This could be an unimplemented feature in clang with respect to C++23 by the
way.

[Bug libstdc++/103295] constexpr std::string does not work for clang

[unlvsur at live dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #4 from cqwrteur <unlvsur at live dot com> ---
(In reply to Andrew Pinski from comment #3)
> This could be an unimplemented feature in clang with respect to C++23 by the
> way.

Same code works with clang --target=x86_64-windows-msvc.

D:\hg\fast_io\tests\0003.concat>clang++ -o constexpr_clang_msvcstl.exe
constexpr.cc -Ofast -std=c++20 -s -flto -march=native -I../../include
--target=x86_64-windows-msvc -fuse-ld=lld -D_DLL=1 -lmsvcrt

[Bug libstdc++/103295] constexpr std::string does not work for clang

[unlvsur at live dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #5 from cqwrteur <unlvsur at live dot com> ---
(In reply to Andrew Pinski from comment #3)
> This could be an unimplemented feature in clang with respect to C++23 by the
> way.

The problem is that it has a silent undefined behavior in the code, the string
does not give the union a storage, the lifetime of the sso buffer is not
activated under constexpr context. (although a proposal is trying to fix that.)

[Bug libstdc++/103295] constexpr std::string does not work for clang

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2021-11-17
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #3)
> This could be an unimplemented feature in clang with respect to C++23 by the
> way.

It's supposed to work in C++20, nothing from 23 should be needed.

[Bug libstdc++/103295] constexpr std::string does not work for clang

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.0

[Bug libstdc++/103295] constexpr std::string does not work for clang

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The code can be reduced to this, which GCC, EDG and MSVC all accept, but Clang
doesn't:

struct S
{
  union {
    char buf[8];
    char* ptr;
  };
  unsigned len;

  constexpr S(const char* s, unsigned n)
  {
    char* p;
    if (n > 7)
      p = ptr = new char[n];
    else
      p = buf;
    for (len = 0; len < n; ++len)
      p[len] = s[len];
    p[len] = '\0';
  }

  constexpr ~S()
  {
    if (len > 7)
      delete[] ptr;
  }
};

constexpr bool test()
{
  S s("test", 4);
  return true;
}

static_assert( test() );


Clang accepts it if written like this:

  constexpr S(const char* s, unsigned n)
  {
    if (n > max_size())
    {
      ptr = new char[n];
      for (len = 0; len < n; ++len)
        ptr[len] = s[len];
      ptr[len] = '\0';
    }
    else
    {
      for (len = 0; len < n; ++len)
        buf[len] = s[len];
      buf[len] = '\0';
    }
  }

It just doesn't allow assignment through the pointer.

[Bug libstdc++/103295] constexpr std::string does not work for clang

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Which means this is enough to make Clang happy for the comment 0 example:

--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -167,6 +167,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _M_construct(_InIterator __beg, _InIterator __end,
                   std::input_iterator_tag)
       {
+#if __cpp_lib_is_constant_evaluated
+       if (__builtin_is_constant_evaluated())
+         _M_local_buf[0] = _CharT();
+#endif
+
        size_type __len = 0;
        size_type __capacity = size_type(_S_local_capacity);

@@ -223,6 +228,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
            _M_data(_M_create(__dnew, size_type(0)));
            _M_capacity(__dnew);
          }
+#if __cpp_lib_is_constant_evaluated
+       else if (__builtin_is_constant_evaluated())
+         _M_local_buf[0] = _CharT();
+#endif

        // Check for out_of_range and length_error exceptions.
        __try
@@ -247,6 +256,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
          _M_data(_M_create(__n, size_type(0)));
          _M_capacity(__n);
        }
+#if __cpp_lib_is_constant_evaluated
+       else if (__builtin_is_constant_evaluated())
+         _M_local_buf[0] = _CharT();
+#endif

       if (__n)
        this->_S_assign(_M_data(), __n, __c);

[Bug libstdc++/103295] constexpr std::string does not work for clang

[unlvsur at live dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #9 from cqwrteur <unlvsur at live dot com> ---
(In reply to Jonathan Wakely from comment #8)
> Which means this is enough to make Clang happy for the comment 0 example:
> 
> --- a/libstdc++-v3/include/bits/basic_string.tcc
> +++ b/libstdc++-v3/include/bits/basic_string.tcc
> @@ -167,6 +167,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        _M_construct(_InIterator __beg, _InIterator __end,
>                    std::input_iterator_tag)
>        {
> +#if __cpp_lib_is_constant_evaluated
> +       if (__builtin_is_constant_evaluated())
> +         _M_local_buf[0] = _CharT();
> +#endif
> +
>         size_type __len = 0;
>         size_type __capacity = size_type(_S_local_capacity);
>  
> @@ -223,6 +228,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>             _M_data(_M_create(__dnew, size_type(0)));
>             _M_capacity(__dnew);
>           }
> +#if __cpp_lib_is_constant_evaluated
> +       else if (__builtin_is_constant_evaluated())
> +         _M_local_buf[0] = _CharT();
> +#endif
>  
>         // Check for out_of_range and length_error exceptions.
>         __try
> @@ -247,6 +256,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>           _M_data(_M_create(__n, size_type(0)));
>           _M_capacity(__n);
>         }
> +#if __cpp_lib_is_constant_evaluated
> +       else if (__builtin_is_constant_evaluated())
> +         _M_local_buf[0] = _CharT();
> +#endif
>  
>        if (__n)
>         this->_S_assign(_M_data(), __n, __c);

where can i download EDG??

[Bug libstdc++/103295] constexpr std::string does not work for clang

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(Do you really need to quote the whole of comment 9 to reply to comment 8?)

You can't, it's not freely available.

[Bug libstdc++/103295] constexpr std::string does not work for clang

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #7)
> It just doesn't allow assignment through the pointer.

I think that's correct according to [class.union.general] p6:

"When the left operand of an assignment operator involves a member access
expression ([expr.ref]) that nominates a union member, it may begin the
lifetime of that union member, as described below."

`_M_local_buf[0] = 0;` is OK, but `auto p = _M_local_buf; p[0] = 0;` is not.

[Bug libstdc++/103295] constexpr std::string does not work for clang

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
But even with those problems fixed, Clang fails for anything that doesn't fit
in the SSO buffer, as shown by this:

#include <memory>

constexpr char test()
{
  char* p = std::allocator<char>().allocate(2);

  p[0] = 'a';
  char c = *p;
  std::allocator<char>().deallocate(p, 2);
  return c;
}

static_assert( test() == 'a' );

[Bug libstdc++/103295] constexpr std::string does not work for clang

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #13 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:6afa1083c6ee7b31629fb0c16299b952cb17868c

commit r12-5343-g6afa1083c6ee7b31629fb0c16299b952cb17868c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Nov 17 10:23:14 2021 +0000

    libstdc++: Set active member of union in std::string [PR103295]

    Clang diagnoses that the new constexpr std::string constructors are not
    usable in constant expressions, because they start to write to members
    of the union without setting an active member.

    This adds a new helper function which returns the address of the local
    buffer after making it the active member.

    This doesn't fix all problems with Clang, because it still refuses to
    write to memory returned by the allocator.

    libstdc++-v3/ChangeLog:

            PR libstdc++/103295
            * include/bits/basic_string.h (_M_use_local_data()): New
            member function to make local buffer the active member.
            (assign(const basic_string&)): Use it.
            * include/bits/basic_string.tcc (_M_construct, reserve()):
            Likewise.

[Bug libstdc++/103295] constexpr std::string does not work for clang

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

Jonathan Wakely <redi at gcc dot gnu.org> changed:

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

--- Comment #14 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I'm going to mark this as fixed, I think the remaining problem is a clang bug.
I'll reopen if Richard Smith convinces me otherwise.

[Bug libstdc++/103295] constexpr std::string does not work for clang

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

Jonathan Wakely <redi at gcc dot gnu.org> changed:

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

--- Comment #15 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reopened :-(

I have a patch to make clang happy, but I think what we really want is a
language change.

[Bug libstdc++/103295] constexpr std::string does not work for clang

[unlvsur at live dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #16 from cqwrteur <unlvsur at live dot com> ---
(In reply to Jonathan Wakely from comment #15)
> Reopened :-(
> 
> I have a patch to make clang happy, but I think what we really want is a
> language change.

hi jwakely. i am trying to add a new patch to ignore copying DLLs to
../$(libdir)/bin if we are build gcc with multilibs (since 32 bits dlls
overrides 64 bits dlls). libgcc does not copy dlls either. do you think that is
appropriate?

[Bug libstdc++/103295] constexpr std::string does not work for clang

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|12.0                        |---

[Bug libstdc++/103295] constexpr std::string does not work for clang

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #17 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:2d76292bd6719d687bc77051da265df8ed7f5a61

commit r12-5413-g2d76292bd6719d687bc77051da265df8ed7f5a61
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Nov 18 10:33:14 2021 +0000

    libstdc++: Begin lifetime of chars in constexpr std::string [PR103295]

    Clang gives errors for constexpr std::string because the memory returned
    by std::allocator<T>::allocate does not contain any objects yet, and
    attempting to set them using char_traits::assign or char_traits::copy
    fails with:

    assignment to object outside its lifetime is not allowed in a constant
expression
                  *__result = *__first;
                            ^
    This adds code to std::char_traits to use std::construct_at to begin
    lifetimes when called during constant evaluation. To support
    specializations of std::basic_string that don't use std::char_traits
    there is now another layer of wrapper around the allocator_traits, so
    that the lifetime of characters is begun as soon as the memory is
    allocated. By doing it in the char traits and allocator traits, the rest
    of basic_string can ignore the problem.

    While modifying char_traits::copy and char_traits::assign to begin
    lifetimes for the constexpr cases, I also replaced their uses of
    std::copy and std::fill_n respectively. That means we don't need
    <bits/stl_algobase.h> for char_traits.

    libstdc++-v3/ChangeLog:

            PR libstdc++/103295
            * include/bits/basic_string.h (_Alloc_traits): Replace typedef
            with struct for C++20 mode.
            * include/bits/basic_string.tcc (_M_replace): Use _Alloc_traits
            for allocation.
            * include/bits/char_traits.h (__gnu_cxx::char_traits::assign):
            Use std::_Construct during constant evaluation.
            (__gnu_cxx::char_traits::assign(CharT*, const CharT*, size_t)):
            Likewise. Replace std::fill_n with memset or manual loop.
            (__gnu_cxx::char_traits::copy): Likewise, replacing std::copy
            with memcpy.
            * include/ext/vstring.h: Include <bits/stl_algobase.h> for
            std::min.
            * include/std/string_view: Likewise.
            *
testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc:
            Add constexpr test.

[Bug libstdc++/103295] constexpr std::string does not work for clang

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #18 from Jonathan Wakely <redi at gcc dot gnu.org> ---
There's still one more fix needed for _M_construct

[Bug libstdc++/103295] constexpr std::string does not work for clang

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #19 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:1f8d01eb1476a997eb1fc686b60fccdf97747faa

commit r12-5421-g1f8d01eb1476a997eb1fc686b60fccdf97747faa
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Nov 19 18:27:59 2021 +0000

    libstdc++: One more change for Clang to support constexpr std::string
[PR103295]

    All writes into the allocated buffer need to be via traits_type::assign
    to begin lifetimes.

    libstdc++-v3/ChangeLog:

            PR libstdc++/103295
            * include/bits/basic_string.tcc (_M_construct): Use the
            traits assign member to write into allcoated memory.

[Bug libstdc++/103295] constexpr std::string does not work for clang

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
   Target Milestone|---                         |12.0
             Status|ASSIGNED                    |RESOLVED

--- Comment #20 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It should be fixed now.

[Bug libstdc++/103295] constexpr std::string does not work for clang

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #21 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:52672be7d328df50f9a05ce3ab44ebcae50fee1b

commit r13-3910-g52672be7d328df50f9a05ce3ab44ebcae50fee1b
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Fri Nov 11 22:23:31 2022 +1100

    libstdc++: Set active union member in constexpr std::string [PR103295]

    Clang still complains about using std::string in constexpr contexts due
    to the changes made in commit 98a0d72a. This patch ensures that we set
    the active member of the union as according to [class.union.general] p6.

    libstdc++-v3/ChangeLog:

            PR libstdc++/103295
            * include/bits/basic_string.h (_M_use_local_data): Set active
            member to _M_local_buf.

    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>

[Bug libstdc++/103295] constexpr std::string does not work for clang

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #22 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:a088d93c210f9b662d706e2fcf63a59d05fe27c1

commit r12-8908-ga088d93c210f9b662d706e2fcf63a59d05fe27c1
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Fri Nov 11 22:23:31 2022 +1100

    libstdc++: Set active union member in constexpr std::string [PR103295]

    Clang still complains about using std::string in constexpr contexts due
    to the changes made in commit 98a0d72a. This patch ensures that we set
    the active member of the union as according to [class.union.general] p6.

    libstdc++-v3/ChangeLog:

            PR libstdc++/103295
            * include/bits/basic_string.h (_M_use_local_data): Set active
            member to _M_local_buf.

    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
    (cherry picked from commit 52672be7d328df50f9a05ce3ab44ebcae50fee1b)