[gcc r11-9918] libstdc++: Ensure C++20 std::stringstream definitions use correct ABI

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

https://gcc.gnu.org/g:615cc91a99aaac737739644639dd9a27b1404b4a

commit r11-9918-g615cc91a99aaac737739644639dd9a27b1404b4a
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Nov 30 22:04:49 2021 +0000

    libstdc++: Ensure C++20 std::stringstream definitions use correct ABI
    
    The definitions of the new C++20 members of std::stringstream etc are
    missing when --with-default-libstdcxx-abi=gcc4-compatible is used,
    because all the explicit instantiations in src/c++20/sstream-inst.cc are
    skipped.
    
    This ensures the contents of that file are compiled with the new ABI, so
    the same set of symbols are exported regardless of which ABI is active
    by default.
    
    libstdc++-v3/ChangeLog:
    
            * src/c++20/sstream-inst.cc (_GLIBCXX_USE_CXX11_ABI): Define to
            select new ABI.
    
    (cherry picked from commit 56b07badf052fb140c4636f6d77cfc02d1ac357c)

Diff:
---
 libstdc++-v3/src/c++20/sstream-inst.cc | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/src/c++20/sstream-inst.cc b/libstdc++-v3/src/c++20/sstream-inst.cc
index b3fbd6ead44..55d1fe5234d 100644
--- a/libstdc++-v3/src/c++20/sstream-inst.cc
+++ b/libstdc++-v3/src/c++20/sstream-inst.cc
@@ -26,7 +26,9 @@
 // ISO C++ 14882:
 //
 
-// Instantiations in this file are only for the new SSO std::string ABI
+// Instantiations in this file are only for the new SSO std::string ABI.
+#define _GLIBCXX_USE_CXX11_ABI 1
+
 #include <sstream>
 
 #if _GLIBCXX_USE_CXX11_ABI