diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index a3a40568fcd..145eaa81edb 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,12 @@
+2018-10-30  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/87784
+	* include/tr2/dynamic_bitset (dynamic_bitset::push_back): When there
+	are no unused bits in the last block, append a new block with the
+	right value so the bit doesn't need to be set. Only increment size
+	after setting the new bit, not before.
+	* testsuite/tr2/dynamic_bitset/pr87784.cc: New test.
+
 2018-10-29  David Malcolm  <dmalcolm@redhat.com>
 
 	* testsuite/17_intro/using_namespace_std_exp_neg.cc: Remove
diff --git a/libstdc++-v3/include/tr2/dynamic_bitset b/libstdc++-v3/include/tr2/dynamic_bitset
index f76c8faf6e3..9e5c8170c81 100644
--- a/libstdc++-v3/include/tr2/dynamic_bitset
+++ b/libstdc++-v3/include/tr2/dynamic_bitset
@@ -727,10 +727,11 @@ namespace tr2
       void
       push_back(bool __bit)
       {
-	if (size_t __offset = this->size() % bits_per_block == 0)
-	  this->_M_do_append_block(block_type(0), this->_M_Nb);
+	if (this->size() % bits_per_block == 0)
+	  this->_M_do_append_block(block_type(__bit), this->_M_Nb);
+	else
+	  this->_M_unchecked_set(this->_M_Nb, __bit);
 	++this->_M_Nb;
-	this->_M_unchecked_set(this->_M_Nb, __bit);
       }
 
       /**
diff --git a/libstdc++-v3/testsuite/tr2/dynamic_bitset/pr87784.cc b/libstdc++-v3/testsuite/tr2/dynamic_bitset/pr87784.cc
new file mode 100644
index 00000000000..52dc3893a31
--- /dev/null
+++ b/libstdc++-v3/testsuite/tr2/dynamic_bitset/pr87784.cc
@@ -0,0 +1,76 @@
+// Copyright (C) 2018 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-do run { target c++11 } }
+
+#include <tr2/dynamic_bitset>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::tr2::dynamic_bitset<unsigned> b;
+  VERIFY( b.size() == 0 );
+  VERIFY( b.find_first() == b.size() );
+  b.push_back(0);
+  VERIFY( b.size() == 1 );
+  VERIFY( b.find_first() == b.size() );
+  b.push_back(0);
+  VERIFY( b.size() == 2 );
+  VERIFY( b.find_first() == b.size() );
+
+  b.push_back(1);
+  VERIFY( b.size() == 3 );
+  VERIFY( b.find_first() == b.size() - 1 );
+  b.push_back(1);
+  VERIFY( b.size() == 4 );
+  VERIFY( b.find_first() == b.size() - 2 );
+  b.push_back(0);
+  VERIFY( b.size() == 5 );
+  VERIFY( b.find_first() == b.size() - 3 );
+
+  b.clear();
+  VERIFY( b.size() == 0 );
+  VERIFY( b.find_first() == b.size() );
+  b.push_back(1);
+  VERIFY( b.size() == 1 );
+  VERIFY( b.find_first() == 0 );
+  b.push_back(1);
+  VERIFY( b.size() == 2 );
+  VERIFY( b.find_first() == 0 );
+  b.push_back(1);
+  VERIFY( b.size() == 3 );
+  VERIFY( b.find_first() == 0 );
+
+  b.clear();
+  b.append(2u);
+  VERIFY( b.size() == b.bits_per_block );
+  VERIFY( b.find_first() == 1 );
+  b <<= 1;
+  VERIFY( b.find_first() == 2 );
+  b <<= 3;
+  VERIFY( b.find_first() == 5 );
+  b <<= 6;
+  VERIFY( b.find_first() == 11 );
+  VERIFY( b.size() == b.bits_per_block );
+}
+
+int
+main()
+{
+  test01();
+}