[Bug libstdc++/96279] build failure: floating_from_chars.cc:310:22: error: '__builtin_isinf_sign' is not a member of 'std'

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96279

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2020-07-22
     Ever confirmed|0                           |1

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
<cmath> has:

#if _GLIBCXX_USE_C99_MATH
#if !_GLIBCXX_USE_C99_FP_MACROS_DYNAMIC

// These are possible macros imported from C99-land.
#undef fpclassify
#undef isfinite
#undef isinf

So for your glibc isinf macro to be preserved we must not be using C99, which
seems lik a bug. glibc-2.31 certainly has a usable C99 implementation, so it
looks like a cross-config bug in libstdc++. It's probably been there for years
and we never noticed until now.