diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index 4d65d2d22af4..59d762976462 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -60,7 +60,7 @@ endif()
 include(CMakeDependentOption)
 
 # Basic options ---------------------------------------------------------------
-option(LIBCXX_ENABLE_ASSERTIONS "Enable assertions independent of build mode." OFF)
+option(LIBCXX_ENABLE_ASSERTIONS "Enable assertions independent of build mode." ON)
 option(LIBCXX_ENABLE_SHARED "Build libc++ as a shared library." ON)
 option(LIBCXX_ENABLE_STATIC "Build libc++ as a static library." ON)
 option(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY "Build libc++experimental.a" ON)
@@ -501,7 +501,6 @@ endif()
 # Assertion flags =============================================================
 define_if(LIBCXX_ENABLE_ASSERTIONS -UNDEBUG)
 define_if_not(LIBCXX_ENABLE_ASSERTIONS -DNDEBUG)
-define_if(LIBCXX_ENABLE_ASSERTIONS -D_LIBCPP_DEBUG=0)
 define_if(LIBCXX_DEBUG_BUILD -D_DEBUG)
 if (LIBCXX_ENABLE_ASSERTIONS AND NOT LIBCXX_DEBUG_BUILD)
   # MSVC doesn't like _DEBUG on release builds. See PR 4379.
diff --git a/libcxx/include/__config b/libcxx/include/__config
index a8a8d57ed40a..44f1344d5713 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -824,9 +824,7 @@ template <unsigned> struct __static_assert_check {};
 #   else
 #       error Supported values for _LIBCPP_DEBUG are 0 and 1
 #   endif
-# if !defined(_LIBCPP_BUILDING_LIBRARY)
 #   define _LIBCPP_EXTERN_TEMPLATE(...)
-# endif
 #endif
 
 #ifndef _LIBCPP_EXTERN_TEMPLATE
diff --git a/libcxx/include/__threading_support b/libcxx/include/__threading_support
index 35b61967edb6..13ab769ba93d 100644
--- a/libcxx/include/__threading_support
+++ b/libcxx/include/__threading_support
@@ -25,6 +25,7 @@
 # include <pthread.h>
 # include <sched.h>
 #elif defined(_LIBCPP_HAS_THREAD_API_WIN32)
+#include <assert.h>
 #include <Windows.h>
 #include <process.h>
 #include <fibersapi.h>
diff --git a/libcxx/src/condition_variable.cpp b/libcxx/src/condition_variable.cpp
index 3f607271b9eb..25e66038eec0 100644
--- a/libcxx/src/condition_variable.cpp
+++ b/libcxx/src/condition_variable.cpp
@@ -14,6 +14,7 @@
 #include "condition_variable"
 #include "thread"
 #include "system_error"
+#include "cassert"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
diff --git a/libcxx/src/experimental/filesystem/path.cpp b/libcxx/src/experimental/filesystem/path.cpp
index daf2c2bba79f..96b81f7b0a70 100644
--- a/libcxx/src/experimental/filesystem/path.cpp
+++ b/libcxx/src/experimental/filesystem/path.cpp
@@ -6,9 +6,11 @@
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+#undef NDEBUG
 #include "experimental/filesystem"
 #include "string_view"
 #include "utility"
+#include "cassert"
 
 namespace { namespace parser
 {
@@ -111,6 +113,7 @@ public:
   void decrement() noexcept {
     const PosPtr REnd = &Path.front() - 1;
     const PosPtr RStart = getCurrentTokenStartPos() - 1;
+    assert(RStart != REnd);
 
     switch (State) {
     case PS_AtEnd: {
@@ -319,6 +322,7 @@ string_view_t path::__root_path_raw() const
       auto NextCh = PP.peek();
       if (NextCh && *NextCh == '/') {
         ++PP;
+        assert(PP.State == PathParser::PS_InRootDir);
         return createView(__pn_.data(), &PP.RawEntry.back());
       }
       return PP.RawEntry;
diff --git a/libcxx/src/mutex.cpp b/libcxx/src/mutex.cpp
index b858e8877ae5..338b79f2feb0 100644
--- a/libcxx/src/mutex.cpp
+++ b/libcxx/src/mutex.cpp
@@ -11,6 +11,7 @@
 #include "mutex"
 #include "limits"
 #include "system_error"
+#include "cassert"
 #include "include/atomic_support.h"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
@@ -44,7 +45,7 @@ mutex::unlock() _NOEXCEPT
 {
     int ec = __libcpp_mutex_unlock(&__m_);
     (void)ec;
-    _LIBCPP_ASSERT(ec == 0, "call to mutex::unlock failed");
+    assert(ec == 0);
 }
 
 // recursive_mutex
@@ -60,7 +61,7 @@ recursive_mutex::~recursive_mutex()
 {
     int e = __libcpp_recursive_mutex_destroy(&__m_);
     (void)e;
-    _LIBCPP_ASSERT(e == 0, "call to ~recursive_mutex() failed");
+    assert(e == 0);
 }
 
 void
@@ -76,7 +77,7 @@ recursive_mutex::unlock() _NOEXCEPT
 {
     int e = __libcpp_recursive_mutex_unlock(&__m_);
     (void)e;
-    _LIBCPP_ASSERT(e == 0, "call to recursive_mutex::unlock() failed");
+    assert(e == 0);
 }
 
 bool
diff --git a/libcxx/src/system_error.cpp b/libcxx/src/system_error.cpp
index cbbbb5dcd15c..94114992c13c 100644
--- a/libcxx/src/system_error.cpp
+++ b/libcxx/src/system_error.cpp
@@ -17,9 +17,9 @@
 #include "cstring"
 #include "cstdio"
 #include "cstdlib"
+#include "cassert"
 #include "string"
 #include "string.h"
-#include "__debug"
 
 #if defined(__ANDROID__)
 #include <android/api-level.h>
@@ -96,7 +96,7 @@ string do_strerror_r(int ev) {
             std::snprintf(buffer, strerror_buff_size, "Unknown error %d", ev);
             return string(buffer);
         } else {
-            _LIBCPP_ASSERT(new_errno == ERANGE, "unexpected error from ::strerr_r");
+            assert(new_errno == ERANGE);
             // FIXME maybe? 'strerror_buff_size' is likely to exceed the
             // maximum error size so ERANGE shouldn't be returned.
             std::abort();