This adjusts the declarations in <charconv> to match when the definition
is present. This solves the issue that std::from_chars is present on
Solaris 11.3 (using fast_float) but was not declared in the header
(because the declarations were guarded by _GLIBCXX_HAVE_USELOCALE).
Additionally, do not define __cpp_lib_to_chars unless both from_chars
and to_chars are supported (which is only true for IEEE float and
double). We might still provide from_chars (via strtold) but if to_chars
isn't provided, we shouldn't define the feature test macro.
Finally, this simplifies some of the preprocessor checks in the bodies
of std::from_chars in src/c++17/floating_from_chars.cc and hoists the
repeated code for the strtod version into a new function template.
N.B. the long double overload of std::from_chars will always be defined
if the float and double overloads are defined. We can always use one of
strtold or fast_float's binary64 routines (although the latter might
produce errors for some long double values if they are not representable
as binary64).
libstdc++-v3/ChangeLog:
* include/std/charconv (__cpp_lib_to_chars): Only define when
both from_chars and to_chars are supported for floating-point
types.
(from_chars, to_chars): Adjust preprocessor conditions guarding
declarations.
* include/std/version (__cpp_lib_to_chars): Adjust condition to
match <charconv> definition.
* src/c++17/floating_from_chars.cc (from_chars_strtod): New
function template.
(from_chars): Simplify preprocessor checks and use
from_chars_strtod when appropriate.
5a4e208022