[Bug libstdc++/107901] New: std::format isn't usable on 16-bit targets

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107901

            Bug ID: 107901
           Summary: std::format isn't usable on 16-bit targets
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

the std::to_chars overloads for floating-point types are gated on
__SIZE_WIDTH__ being at least 32-bits:

#if _GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64 \
    && __SIZE_WIDTH__ >= 32 && _GLIBCXX_HOSTED
# define __cpp_lib_to_chars 201611L
#endif
...
#if defined __cpp_lib_to_chars
  // Floating-point std::to_chars

  // Overloads for float.
  to_chars_result to_chars(char* __first, char* __last, float __value)
noexcept;
  to_chars_result to_chars(char* __first, char* __last, float __value,
                           chars_format __fmt) noexcept;
  to_chars_result to_chars(char* __first, char* __last, float __value,
                           chars_format __fmt, int __precision) noexcept;


This means that std::format fails to compile on targets with 16-bit size_t,
because it assumes that FP types can be printed via std::to_chars:

/home/jwakely/src/gcc/build-16bit/msp430-elf/libstdc++-v3/include/format: In
instantiation of
'std::__format::_Formatting_scanner<std::__format::_Sink_iter<wchar_t>,
wchar_t>::_M_format_arg(std::size_t)::<lambda(auto:42&)> [with auto:42 =
float]':
/home/jwakely/src/gcc/build-16bit/msp430-elf/libstdc++-v3/include/format:3038:44:
  required from 'decltype(auto)
std::basic_format_arg<_Context>::_M_visit(_Visitor&&, std::__format::_Arg_t)
[with _Visitor =
std::__format::_Formatting_scanner<std::__format::_Sink_iter<wchar_t>,
wchar_t>::_M_format_arg(std::size_t)::<lambda(auto:42&)>; _Context =
std::basic_format_context<std::__format::_Sink_iter<wchar_t>, wchar_t>]'
/home/jwakely/src/gcc/build-16bit/msp430-elf/libstdc++-v3/include/format:3082:28:
  required from 'decltype(auto) std::visit_format_arg(_Visitor&&,
basic_format_arg<_Context>) [with _Visitor =
__format::_Formatting_scanner<__format::_Sink_iter<wchar_t>,
wchar_t>::_M_format_arg(std::size_t)::<lambda(auto:42&)>; _Context =
basic_format_context<__format::_Sink_iter<wchar_t>, wchar_t>]'
/home/jwakely/src/gcc/build-16bit/msp430-elf/libstdc++-v3/include/format:3532:23:
  required from 'void std::__format::_Formatting_scanner<_Out,
_CharT>::_M_format_arg(std::size_t) [with _Out =
std::__format::_Sink_iter<wchar_t>; _CharT = wchar_t; std::size_t = unsigned
int]'
/home/jwakely/src/gcc/build-16bit/msp430-elf/libstdc++-v3/include/format:3527:7:
  required from here
/home/jwakely/src/gcc/build-16bit/msp430-elf/libstdc++-v3/include/format:3546:37:
error: static assertion failed
 3546 |             static_assert(__format::__formattable_with<_Type,
_Context>);
      |                           ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Long term, we want to support std::to_chars everywhere. In the short term, we
might want to disable FP types in std::format so that other types can still be
formatted.