[gcc r16-772] libstdc++: use maintained size when split pb_ds binary search trees

[Xi Ruoyao]

https://gcc.gnu.org/g:36c20fee22d40c6d25f52e929b42f5eab62cb1eb

commit r16-772-g36c20fee22d40c6d25f52e929b42f5eab62cb1eb
Author: Xℹ Ruoyao <xry111@mengyan1223.wang>
Date:   Fri Jul 10 21:38:09 2020 +0800

    libstdc++: use maintained size when split pb_ds binary search trees
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/81806
            * include/ext/pb_ds/detail/bin_search_tree_/split_join_fn_imps.hpp
            (split_finish): Use maintained size, instead of calling
            std::distance.

Diff:
---
 .../include/ext/pb_ds/detail/bin_search_tree_/split_join_fn_imps.hpp  | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/ext/pb_ds/detail/bin_search_tree_/split_join_fn_imps.hpp b/libstdc++-v3/include/ext/pb_ds/detail/bin_search_tree_/split_join_fn_imps.hpp
index 0c1b26fa9e2d..a2a57757a046 100644
--- a/libstdc++-v3/include/ext/pb_ds/detail/bin_search_tree_/split_join_fn_imps.hpp
+++ b/libstdc++-v3/include/ext/pb_ds/detail/bin_search_tree_/split_join_fn_imps.hpp
@@ -133,7 +133,9 @@ PB_DS_CLASS_C_DEC::
 split_finish(PB_DS_CLASS_C_DEC& other)
 {
   other.initialize_min_max();
-  other.m_size = std::distance(other.begin(), other.end());
+  other.m_size = 0;
+  if (other.m_p_head->m_p_parent != 0)
+    other.m_size = other.m_p_head->m_p_parent->m_subtree_size;
   m_size -= other.m_size;
   initialize_min_max();
   PB_DS_ASSERT_VALID((*this))