libstdc++: Ensure that std::from_chars is declared when supported

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.

1 files changed, 2 insertions, 1 deletions

diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index 461e65b..d730a7e 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -171,7 +171,8 @@
 #endif
 #define __cpp_lib_shared_ptr_weak_type 201606L
 #define __cpp_lib_string_view 201803L
-#if _GLIBCXX_HAVE_USELOCALE
+#if _GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64 \
+    && __SIZE_WIDTH__ >= 32
 # define __cpp_lib_to_chars 201611L
 #endif
 #define __cpp_lib_unordered_map_try_emplace 201411L