[PATCH] libstdc++: Ensure that std::from_chars is declared when supported

[Jonathan Wakely]

On Mon, 14 Mar 2022 at 14:17, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> On Fri, 11 Mar 2022, Jonathan Wakely wrote:
>
> > Patrick, I think this is right, but please take a look to double check.
> >
> > I think we should fix the feature-test macro conditions for gcc-11 too,
> > although it's a bit more complicated there. It should depend on IEEE
> > float and double *and* uselocale. We don't need the other changes on the
> > branch.
>
> Don't we still depend on uselocale in GCC 12 for long double from_chars,
> at least on targets where long double != binary64?

Not after this patch:

from_chars(const char* first, const char* last, long double& value,
          chars_format fmt) noexcept
{
-#if _GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64 \
-  && ! USE_STRTOD_FOR_FROM_CHARS
+#if ! USE_STRTOD_FOR_FROM_CHARS
+  // Either long double is the same as double, or we can't use strtold.
+  // In the latter case, this might give an incorrect result (e.g. values
+  // out of range of double give an error, even if they fit in long double).

If uselocale isn't available, this defines the long double overload in
terms of the double one, even if that doesn't always give the right
answers. That greatly simplifies the preprocessor conditions for when
it's supported. If the float and double forms are present, so is the
long double one.