[Bug libstdc++/110917] New: std::format_to(int*, ...) fails to compile because of _S_make_span

[Bug libstdc++/110917] New: std::format_to(int*, ...) fails to compile because of _S_make_span

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110917

            Bug ID: 110917
           Summary: std::format_to(int*, ...) fails to compile because of
                    _S_make_span
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arthur.j.odwyer at gmail dot com
  Target Milestone: ---

// https://godbolt.org/z/9onGvqfha
#include <format>
#include <list>
#include <vector>

void f(std::list<int>::iterator it) {
    std::format_to(it, "");  // OK
}
void g(std::vector<int>::iterator it) {
    std::format_to(it, "");  // boom!
}

libc++ and Microsoft are both completely happy with this code. libstdc++ is
happy with `f` but unhappy with `g`. I suspect that someplace is accepting
"contiguous iterators of any old T" when it means to limit itself to
"contiguous iterators of char" specifically.


In file included from <source>:1:
.../include/c++/14.0.0/format:2609:36: error: cannot initialize a parameter of
type 'char *' with an rvalue of type 'int *'
 2609 |       : _Sink<_CharT>(_S_make_span(std::to_address(__out), __n,
_M_buf)),
      |                                    ^~~~~~~~~~~~~~~~~~~~~~
.../include/c++/14.0.0/format:3641:32: note: in instantiation of member
function 'std::__format::_Iter_sink<char, __gnu_cxx::__normal_iterator<int *,
std::vector<int>>>::_Iter_sink' requested here
 3641 |       _Iter_sink<_CharT, _Out> __sink(std::move(__out));
      |                                ^
.../include/c++/14.0.0/format:3683:24: note: in instantiation of function
template specialization
'std::__format::__do_vformat_to<__gnu_cxx::__normal_iterator<int *,
std::vector<int>>, char,
std::basic_format_context<std::__format::_Sink_iter<char>, char>>' requested
here
 3683 |     { return __format::__do_vformat_to(std::move(__out), __fmt,
__args); }
      |                        ^
.../include/c++/14.0.0/format:3778:19: note: in instantiation of function
template specialization 'std::vformat_to<__gnu_cxx::__normal_iterator<int *,
std::vector<int>>>' requested here
 3778 |       return std::vformat_to(std::move(__out), __fmt.get(),
      |                   ^
<source>:9:10: note: in instantiation of function template specialization
'std::format_to<__gnu_cxx::__normal_iterator<int *, std::vector<int>>>'
requested here
    9 |     std::format_to(it, "");
      |          ^
.../include/c++/14.0.0/format:2574:28: note: passing argument to parameter
'__ptr' here
 2574 |       _S_make_span(_CharT* __ptr, iter_difference_t<_OutIter> __n,
      |                            ^
1 error generated.

[Bug libstdc++/110917] std::format_to(int*, ...) fails to compile because of _S_make_span

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110917

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
   Last reconfirmed|                            |2023-08-06
   Target Milestone|---                         |13.3
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Arthur O'Dwyer from comment #0)
> libc++ and Microsoft are both completely happy with this code. libstdc++ is
> happy with `f` but unhappy with `g`. I suspect that someplace is accepting
> "contiguous iterators of any old T" when it means to limit itself to
> "contiguous iterators of char" specifically.

Indeed, it's fixed by:

   // not introduce any invalid pointer arithmetic or overflows that would not
   // have happened anyway.
   template<typename _CharT, contiguous_iterator _OutIter>
+    requires same_as<iter_value_t<_OutIter>, _CharT>
     class _Iter_sink<_CharT, _OutIter> : public _Sink<_CharT>
     {
       using uint64_t = __UINTPTR_TYPE__;

Alternatively, we could replace the contiguous_iterator<_OutIter> constraint
with constructible_from<span<_CharT>, _OutIter, iter_difference_t<_OutIter>>.

[Bug libstdc++/110917] std::format_to(int*, ...) fails to compile because of _S_make_span

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110917

--- Comment #2 from Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> ---
> Alternatively, we could replace the contiguous_iterator<_OutIter> constraint with constructible_from<span<_CharT>, _OutIter, iter_difference_t<_OutIter>>.

I think `is_same` is preferable to `constructible_from<...>` just because it's
simpler; I wouldn't recommend the hairier thing unless there was a known reason
for it. Doing the hairier thing would immediately trigger a search for the
corner case where it would fail. ;)  (Suppose the author of _OutIter arranges
that sentinel_for<size_t, _OutIter> — maybe that'd do the trick...)


Btw, even though my reduced test case was contrived, it originates from an
actually realistic use-case AFAIK: using `format` to format ASCII text
automatically into an array of char16_t or char32_t (presumably on a platform
with unsigned plain char).

[Bug libstdc++/110917] std::format_to(int*, ...) fails to compile because of _S_make_span

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110917

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It fails for non-contrived cases like this too:

char8_t buf[32];
std::format_to(buf, "");

[Bug libstdc++/110917] std::format_to(int*, ...) fails to compile because of _S_make_span

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110917

--- Comment #4 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:c5ea5aecac323e9094e4dc967f54090cb244bc6a

commit r14-3068-gc5ea5aecac323e9094e4dc967f54090cb244bc6a
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Aug 7 14:37:25 2023 +0100

    libstdc++: Constrain __format::_Iter_sink for contiguous iterators
[PR110917]

    We can't write to a span<_CharT> if the contiguous iterator has a value
    type that isn't _CharT.

    libstdc++-v3/ChangeLog:

            PR libstdc++/110917
            * include/std/format (__format::_Iter_sink<CharT, OutIter>):
            Constrain partial specialization for contiguous iterators to
            require the value type to be CharT.
            * testsuite/std/format/functions/format_to.cc: New test.

[Bug libstdc++/110917] std::format_to(int*, ...) fails to compile because of _S_make_span

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110917

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-13 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:0f0152a93d15b24ebc7f6c7455baaded6a63fb2e

commit r13-7698-g0f0152a93d15b24ebc7f6c7455baaded6a63fb2e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Aug 7 14:37:25 2023 +0100

    libstdc++: Constrain __format::_Iter_sink for contiguous iterators
[PR110917]

    We can't write to a span<_CharT> if the contiguous iterator has a value
    type that isn't _CharT.

    libstdc++-v3/ChangeLog:

            PR libstdc++/110917
            * include/std/format (__format::_Iter_sink<CharT, OutIter>):
            Constrain partial specialization for contiguous iterators to
            require the value type to be CharT.
            * testsuite/std/format/functions/format_to.cc: New test.

    (cherry picked from commit c5ea5aecac323e9094e4dc967f54090cb244bc6a)

[Bug libstdc++/110917] std::format_to(int*, ...) fails to compile because of _S_make_span

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110917

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

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

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for 13.3, thanks for the report.