[gcc r15-10521] libstdc++: Fix std module for gcc4-compatible ABI

[Jonathan Wakely]

https://gcc.gnu.org/g:e46321811a95faebb38a3996cec7317df3fa248a

commit r15-10521-ge46321811a95faebb38a3996cec7317df3fa248a
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Nov 17 14:33:50 2025 +0000

    libstdc++: Fix std module for gcc4-compatible ABI
    
    Add preprocessor checks to std.cc.in for features which are not
    supported with the old string ABI.
    
    The COW std::string doesn't properly support C++11 allocators, so the
    aliases such as std::pmr::string, std::pmr::u8string etc. are not
    defined for the old string ABI.
    
    The std::syncbuf type uses std::string and is not defined for the old
    string ABI.
    
    libstdc++-v3/ChangeLog:
    
            * src/c++23/std.cc.in [!__cpp_lib_syncbuf]: Disable exports for
            <syncstream> contents when not defined.
            [!_GLIBCXX_USE_CXX11_ABI]: Disable exports for pmr aliases in
            <string> when not defined.
    
    (cherry picked from commit edde0ae2b5299aeef5555346788d9059de45d813)

Diff:
---
 libstdc++-v3/src/c++23/std.cc.in | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/libstdc++-v3/src/c++23/std.cc.in b/libstdc++-v3/src/c++23/std.cc.in
index 5df25bb646af..36e4361e2a3f 100644
--- a/libstdc++-v3/src/c++23/std.cc.in
+++ b/libstdc++-v3/src/c++23/std.cc.in
@@ -1575,15 +1575,17 @@ export namespace std
   using std::u32streampos;
   using std::u8streampos;
   using std::wstreampos;
-  using std::basic_osyncstream;
-  using std::basic_syncbuf;
   using std::istreambuf_iterator;
   using std::ostreambuf_iterator;
+  using std::fpos;
+#ifdef __cpp_lib_syncbuf
+  using std::basic_osyncstream;
+  using std::basic_syncbuf;
   using std::osyncstream;
   using std::syncbuf;
   using std::wosyncstream;
   using std::wsyncbuf;
-  using std::fpos;
+#endif
 }
 
 // <iostream>
@@ -2116,9 +2118,11 @@ export namespace std
   using std::ostream;
   using std::wostream;
   using std::operator<<;
+#ifdef __cpp_lib_syncbuf
   using std::emit_on_flush;
   using std::noemit_on_flush;
   using std::flush_emit;
+#endif
 }
 
 // <print>
@@ -2559,11 +2563,13 @@ export namespace std
   using std::wsregex_token_iterator;
   namespace pmr
   {
+#if _GLIBCXX_USE_CXX11_ABI
     using std::pmr::cmatch;
     using std::pmr::match_results;
     using std::pmr::smatch;
     using std::pmr::wcmatch;
     using std::pmr::wsmatch;
+#endif
   }
 }
 
@@ -2789,12 +2795,14 @@ export namespace std
   using std::wstring;
   namespace pmr
   {
+#if _GLIBCXX_USE_CXX11_ABI
     using std::pmr::basic_string;
     using std::pmr::string;
     using std::pmr::u16string;
     using std::pmr::u32string;
     using std::pmr::u8string;
     using std::pmr::wstring;
+#endif
   }
   using std::hash;
 }
@@ -2839,6 +2847,7 @@ export namespace std
 // <syncstream>
 export namespace std
 {
+#ifdef __cpp_lib_syncbuf
   using std::basic_syncbuf;
   using std::swap;
   using std::basic_osyncstream;
@@ -2846,6 +2855,7 @@ export namespace std
   using std::syncbuf;
   using std::wosyncstream;
   using std::wsyncbuf;
+#endif
 }
 
 // 19.5 <system_error>