[gcc r14-1451] libstdc++: Fix build for targets without _Float128 [PR109921]

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

https://gcc.gnu.org/g:a239a35075ffd8b34f1db72c22998a625ff962b5

commit r14-1451-ga239a35075ffd8b34f1db72c22998a625ff962b5
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed May 31 18:01:13 2023 +0100

    libstdc++: Fix build for targets without _Float128 [PR109921]
    
    My r14-1431-g7037e7b6e4ac41 change caused the _Float128 overload to be
    compiled unconditionally, by moving the USE_STRTOF128_FOR_FROM_CHARS
    check into the function body. That function should still only be
    compiled if the target actually supports _Float128.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/109921
            * src/c++17/floating_from_chars.cc: Check __FLT128_MANT_DIG__ is
            defined before trying to use _Float128.

Diff:
---
 libstdc++-v3/src/c++17/floating_from_chars.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/src/c++17/floating_from_chars.cc b/libstdc++-v3/src/c++17/floating_from_chars.cc
index eea878072b0..f1dd1037bf3 100644
--- a/libstdc++-v3/src/c++17/floating_from_chars.cc
+++ b/libstdc++-v3/src/c++17/floating_from_chars.cc
@@ -1325,7 +1325,7 @@ _ZSt10from_charsPKcS0_RDF128_St12chars_format(const char* first,
 					      __ieee128& value,
 					      chars_format fmt) noexcept
 __attribute__((alias ("_ZSt10from_charsPKcS0_Ru9__ieee128St12chars_format")));
-#else
+#elif defined(__FLT128_MANT_DIG__)
 from_chars_result
 from_chars(const char* first, const char* last, _Float128& value,
 	   chars_format fmt) noexcept