[Bug libstdc++/110077] [14 regression] libstdc++-abi/abi_check FAILs on Solaris

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110077

--- Comment #15 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I now think the right fix is:

--- a/libstdc++-v3/src/c++17/floating_from_chars.cc
+++ b/libstdc++-v3/src/c++17/floating_from_chars.cc
@@ -1325,24 +1325,14 @@ _ZSt10from_charsPKcS0_RDF128_St12chars_format(const
char* first,
                                              __ieee128& value,
                                              chars_format fmt) noexcept
 __attribute__((alias ("_ZSt10from_charsPKcS0_Ru9__ieee128St12chars_format")));
-#elif __FLT128_MANT_DIG__ == 113 && __LDBL_MANT_DIG__ != 113
+#elif defined(USE_STRTOF128_FOR_FROM_CHARS)
 // Overload for _Float128 is not defined inline in <charconv>, define it here.
 from_chars_result
 from_chars(const char* first, const char* last, _Float128& value,
           chars_format fmt) noexcept
 {
-#ifdef USE_STRTOF128_FOR_FROM_CHARS
   // fast_float doesn't support IEEE binary128 format, but we can use strtold.
   return from_chars_strtod(first, last, value, fmt);
-#else
-  // Read a long double. This might give an incorrect result (e.g. values
-  // out of range of long double give an error, even if they fit in
_Float128).
-  long double ldbl_val;
-  auto res = std::from_chars(first, last, ldbl_val, fmt);
-  if (res.ec == errc{})
-    value = ldbl_val;
-  return res;
-#endif
 }
 #endif


I think r14-1431-g7037e7b6e4ac41 was wrong to try to define that overload
unconditionally.  Not all targets need it, so defining the lossy fallback using
long double is not useful (and caused this ABI change on Solaris x86).