diff --git a/libcxx/src/strstream.cpp b/libcxx/src/strstream.cpp
index ea728138db66..83702fc72e80 100644
--- a/libcxx/src/strstream.cpp
+++ b/libcxx/src/strstream.cpp
@@ -11,6 +11,7 @@
 #include "algorithm"
 #include "climits"
 #include "cstring"
+#include "__debug"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
@@ -167,7 +168,10 @@ strstreambuf::overflow(int_type __c)
             buf = new char[new_size];
         if (buf == nullptr)
             return int_type(EOF);
-        memcpy(buf, eback(), static_cast<size_t>(old_size));
+        if (old_size != 0) {
+            _LIBCPP_ASSERT(eback(), "overflow copying from NULL");
+            memcpy(buf, eback(), static_cast<size_t>(old_size));
+        }
         ptrdiff_t ninp = gptr()  - eback();
         ptrdiff_t einp = egptr() - eback();
         ptrdiff_t nout = pptr()  - pbase();
diff --git a/libcxx/test/libcxx/test/config.py b/libcxx/test/libcxx/test/config.py
index 4d4eb066e026..5632e535d4b7 100644
--- a/libcxx/test/libcxx/test/config.py
+++ b/libcxx/test/libcxx/test/config.py
@@ -620,12 +620,13 @@ class Configuration(object):
                 blacklist = os.path.join(self.libcxx_src_root,
                                          'test/ubsan_blacklist.txt')
                 self.cxx.flags += ['-fsanitize=undefined',
-                                   '-fno-sanitize=vptr,function',
+                                   '-fno-sanitize=vptr,function,float-divide-by-zero',
                                    '-fno-sanitize-recover=all',
                                    '-fsanitize-blacklist=' + blacklist]
                 self.cxx.compile_flags += ['-O3']
                 self.env['UBSAN_OPTIONS'] = 'print_stacktrace=1'
                 self.config.available_features.add('ubsan')
+                self.config.available_features.add('sanitizer-new-delete')
             elif san == 'Thread':
                 self.cxx.flags += ['-fsanitize=thread']
                 self.config.available_features.add('tsan')
diff --git a/libcxx/test/std/atomics/atomics.flag/atomic_flag_clear.pass.cpp b/libcxx/test/std/atomics/atomics.flag/atomic_flag_clear.pass.cpp
index 64093d639e44..22bbbd6af535 100644
--- a/libcxx/test/std/atomics/atomics.flag/atomic_flag_clear.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.flag/atomic_flag_clear.pass.cpp
@@ -23,12 +23,14 @@ int main()
 {
     {
         std::atomic_flag f;
+        f.clear();
         f.test_and_set();
         atomic_flag_clear(&f);
         assert(f.test_and_set() == 0);
     }
     {
         volatile std::atomic_flag f;
+        f.clear();
         f.test_and_set();
         atomic_flag_clear(&f);
         assert(f.test_and_set() == 0);
diff --git a/libcxx/test/std/atomics/atomics.flag/atomic_flag_clear_explicit.pass.cpp b/libcxx/test/std/atomics/atomics.flag/atomic_flag_clear_explicit.pass.cpp
index e1a9349c9394..1a212c6f352a 100644
--- a/libcxx/test/std/atomics/atomics.flag/atomic_flag_clear_explicit.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.flag/atomic_flag_clear_explicit.pass.cpp
@@ -22,38 +22,44 @@
 int main()
 {
     {
-        std::atomic_flag f;
-        f.test_and_set();
+        std::atomic_flag f; // uninitialized first
+        atomic_flag_clear_explicit(&f, std::memory_order_relaxed);
+        assert(f.test_and_set() == 0);
         atomic_flag_clear_explicit(&f, std::memory_order_relaxed);
         assert(f.test_and_set() == 0);
     }
     {
         std::atomic_flag f;
-        f.test_and_set();
+        atomic_flag_clear_explicit(&f, std::memory_order_release);
+        assert(f.test_and_set() == 0);
         atomic_flag_clear_explicit(&f, std::memory_order_release);
         assert(f.test_and_set() == 0);
     }
     {
         std::atomic_flag f;
-        f.test_and_set();
+        atomic_flag_clear_explicit(&f, std::memory_order_seq_cst);
+        assert(f.test_and_set() == 0);
         atomic_flag_clear_explicit(&f, std::memory_order_seq_cst);
         assert(f.test_and_set() == 0);
     }
     {
         volatile std::atomic_flag f;
-        f.test_and_set();
+        atomic_flag_clear_explicit(&f, std::memory_order_relaxed);
+        assert(f.test_and_set() == 0);
         atomic_flag_clear_explicit(&f, std::memory_order_relaxed);
         assert(f.test_and_set() == 0);
     }
     {
         volatile std::atomic_flag f;
-        f.test_and_set();
+        atomic_flag_clear_explicit(&f, std::memory_order_release);
+        assert(f.test_and_set() == 0);
         atomic_flag_clear_explicit(&f, std::memory_order_release);
         assert(f.test_and_set() == 0);
     }
     {
         volatile std::atomic_flag f;
-        f.test_and_set();
+        atomic_flag_clear_explicit(&f, std::memory_order_seq_cst);
+        assert(f.test_and_set() == 0);
         atomic_flag_clear_explicit(&f, std::memory_order_seq_cst);
         assert(f.test_and_set() == 0);
     }
diff --git a/libcxx/test/std/atomics/atomics.flag/clear.pass.cpp b/libcxx/test/std/atomics/atomics.flag/clear.pass.cpp
index 65051af790d2..255af8f176ea 100644
--- a/libcxx/test/std/atomics/atomics.flag/clear.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.flag/clear.pass.cpp
@@ -22,50 +22,58 @@
 int main()
 {
     {
-        std::atomic_flag f;
-        f.test_and_set();
+        std::atomic_flag f; // uninitialized
+        f.clear();
+        assert(f.test_and_set() == 0);
         f.clear();
         assert(f.test_and_set() == 0);
     }
     {
         std::atomic_flag f;
-        f.test_and_set();
+        f.clear(std::memory_order_relaxed);
+        assert(f.test_and_set() == 0);
         f.clear(std::memory_order_relaxed);
         assert(f.test_and_set() == 0);
     }
     {
         std::atomic_flag f;
-        f.test_and_set();
+        f.clear(std::memory_order_release);
+        assert(f.test_and_set() == 0);
         f.clear(std::memory_order_release);
         assert(f.test_and_set() == 0);
     }
     {
         std::atomic_flag f;
-        f.test_and_set();
+        f.clear(std::memory_order_seq_cst);
+        assert(f.test_and_set() == 0);
         f.clear(std::memory_order_seq_cst);
         assert(f.test_and_set() == 0);
     }
     {
         volatile std::atomic_flag f;
-        f.test_and_set();
+        f.clear();
+        assert(f.test_and_set() == 0);
         f.clear();
         assert(f.test_and_set() == 0);
     }
     {
         volatile std::atomic_flag f;
-        f.test_and_set();
+        f.clear(std::memory_order_relaxed);
+        assert(f.test_and_set() == 0);
         f.clear(std::memory_order_relaxed);
         assert(f.test_and_set() == 0);
     }
     {
         volatile std::atomic_flag f;
-        f.test_and_set();
+        f.clear(std::memory_order_release);
+        assert(f.test_and_set() == 0);
         f.clear(std::memory_order_release);
         assert(f.test_and_set() == 0);
     }
     {
         volatile std::atomic_flag f;
-        f.test_and_set();
+        f.clear(std::memory_order_seq_cst);
+        assert(f.test_and_set() == 0);
         f.clear(std::memory_order_seq_cst);
         assert(f.test_and_set() == 0);
     }
diff --git a/libcxx/test/std/experimental/algorithms/alg.random.sample/sample.pass.cpp b/libcxx/test/std/experimental/algorithms/alg.random.sample/sample.pass.cpp
index 0f0784d6ee90..1a9f9b099b20 100644
--- a/libcxx/test/std/experimental/algorithms/alg.random.sample/sample.pass.cpp
+++ b/libcxx/test/std/experimental/algorithms/alg.random.sample/sample.pass.cpp
@@ -64,12 +64,12 @@ void test() {
   end = std::experimental::sample(PopulationIterator(ia),
                                   PopulationIterator(ia + is),
                                   SampleIterator(oa), os, g);
-  assert(&*end - oa == std::min(os, is));
+  assert(end.base() - oa == std::min(os, is));
   assert(std::equal(oa, oa + os, oa1));
   end = std::experimental::sample(PopulationIterator(ia),
                                   PopulationIterator(ia + is),
                                   SampleIterator(oa), os, g);
-  assert(&*end - oa == std::min(os, is));
+  assert(end.base() - oa == std::min(os, is));
   assert(std::equal(oa, oa + os, oa2));
 }
 
@@ -85,7 +85,7 @@ void test_empty_population() {
   SampleIterator end =
       std::experimental::sample(PopulationIterator(ia), PopulationIterator(ia),
                                 SampleIterator(oa), os, g);
-  assert(&*end == oa);
+  assert(end.base() == oa);
 }
 
 template <template<class> class PopulationIteratorType, class PopulationItem,
@@ -100,7 +100,7 @@ void test_empty_sample() {
   SampleIterator end =
       std::experimental::sample(PopulationIterator(ia), PopulationIterator(ia + is),
                                 SampleIterator(oa), 0, g);
-  assert(&*end == oa);
+  assert(end.base() == oa);
 }
 
 template <template<class> class PopulationIteratorType, class PopulationItem,
@@ -119,8 +119,8 @@ void test_small_population() {
   end = std::experimental::sample(PopulationIterator(ia),
                                   PopulationIterator(ia + is),
                                   SampleIterator(oa), os, g);
-  assert(&*end - oa == std::min(os, is));
-  assert(std::equal(oa, &*end, oa1));
+  assert(end.base() - oa == std::min(os, is));
+  assert(std::equal(oa, end.base(), oa1));
 }
 
 int main() {