[Bug libstdc++/101542] __gnu_cxx::sequence_buffer const copy constructor is badly broken

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101542

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This should work:

--- a/libstdc++-v3/include/ext/rope
+++ b/libstdc++-v3/include/ext/rope
@@ -203,6 +203,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        std::copy(__x._M_buffer, __x._M_buffer + __x._M_buf_count, _M_buffer);
       }

+      // Non-const "copy" modifies the parameter - yuck
       sequence_buffer(sequence_buffer& __x)
       {
        __x.flush();
@@ -213,6 +214,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       sequence_buffer(_Sequence& __s)
       : _M_prefix(&__s), _M_buf_count(0) { }

+      // Non-const "copy" modifies the parameter - yuck
       sequence_buffer&
       operator=(sequence_buffer& __x)
       {
@@ -230,7 +232,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        std::copy(__x._M_buffer, __x._M_buffer + __x._M_buf_count, _M_buffer);
        return *this;
       }
-      
+
+#if __cplusplus >= 201103L
+      sequence_buffer(sequence_buffer&& __x) : sequence_buffer(__x) { }
+      sequence_buffer& operator=(sequence_buffer&& __x) { return *this = __x;
}
+#endif
+
       void
       push_back(value_type __x)
       {