[gcc r14-9879] libstdc++: Fix build for targets without FP std::from_chars [PR114633]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:92b38ec84f2990d217f036dc6c5a32cde5ecfb93

commit r14-9879-g92b38ec84f2990d217f036dc6c5a32cde5ecfb93
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Apr 8 17:37:32 2024 +0100

    libstdc++: Fix build for targets without FP std::from_chars [PR114633]
    
    If the faster std::from_chars is not supported for floating-point types
    then just extract the value from the stream using operator>>.
    
    This fixes a build error for targets where __cpp_lib_to_chars is not
    defined.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/114633
            * include/bits/chrono_io.h (_Parser::operator()) <'S'>: Use
            stream extraction if std::from_chars is not available.

Diff:
---
 libstdc++-v3/include/bits/chrono_io.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
index b9eb3d2be53..3b34992b42a 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -3685,6 +3685,7 @@ namespace __detail
 		      if (!__is_failed(__err)) [[likely]]
 			{
 			  long double __val{};
+#if __cpp_lib_to_chars
 			  string __str = std::move(__buf).str();
 			  auto __first = __str.data();
 			  auto __last = __first + __str.size();
@@ -3694,6 +3695,9 @@ namespace __detail
 			  if ((bool)ec || ptr != __last) [[unlikely]]
 			    __err |= ios_base::failbit;
 			  else
+#else
+			  if (__buf >> __val)
+#endif
 			    {
 			      duration<long double> __fs(__val);
 			      if constexpr (__is_floating)