On Sun, 29 Dec 2024 at 14:57, Hans Åberg <haberg_1@xxxxxxxxxx> wrote:
>
>
> > On 29 Dec 2024, at 15:16, Jonathan Wakely <jwakely.gcc@xxxxxxxxx> wrote:
> >
> > On Sun, 29 Dec 2024 at 10:01, Hans Åberg via Gcc-help
> > <gcc-help@xxxxxxxxxxx> wrote:
> >>
> >> When checking the GCC 14.2 implementation of C++23 fixed width floating-point types in the header <stdfloat> [1]:
> >>
> >> The page [2] says that one should be able to write std::float128_t using std::formatter as it is using std::to_chars which is required to support all arithmetic type, but it does not work with GCC.
> >
> > It works fine for me.
> >
> >> But the wording suggests it has been working with GCC.
> >
> > And still is working.
> >
> > If it's not working for you, it must be a limitation of your OS, which
> > you haven't told us.
>
> It is MacOS 15.2, Silicon, but I think the error is the same on Intel, with
> % g++ --version
> g++ (MacPorts gcc14 14.2.0_3+stdlib_flag) 14.2.0
That's the problem then. std::to_chars is only defined for float128
under these conditions:
#if defined(__STDCPP_FLOAT128_T__) &&
defined(_GLIBCXX_LDOUBLE_IS_IEEE_BINARY128)
...
#elif defined(__STDCPP_FLOAT128_T__) && defined(_GLIBCXX_HAVE_FLOAT128_MATH)
...
#endif
The HAVE_FLOAT128_MATH macro is currently only defined for Glibc 2.26
and later, on targets that support __float128.
Maybe it would make the errors clearer if we did this:
--- a/libstdc++-v3/include/std/charconv
+++ b/libstdc++-v3/include/std/charconv
@@ -913,6 +913,11 @@ namespace __detail
to_chars_result to_chars(char* __first, char* __last, _Float128 __value,
chars_format __fmt, int __precision) noexcept;
#endif
+#else
+ // No libc support for formatting 128-bit floating-point types.
+ to_chars_result to_chars(char*, char*, _Float128) = delete;
+ to_chars_result to_chars(char*, char*, _Float128, chars_format) = delete;
+ to_chars_result to_chars(char*, char*, _Float128, chars_format,
int) = delete;
#endif
