[libc++] Implementation of C++20's P1135R6 for libcxx

Differential Revision: https://reviews.llvm.org/D68480
diff --git a/libcxx/test/std/thread/thread.barrier/completion.pass.cpp b/libcxx/test/std/thread/thread.barrier/completion.pass.cpp
new file mode 100644
index 0000000..67ae526
--- /dev/null
+++ b/libcxx/test/std/thread/thread.barrier/completion.pass.cpp
@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03, c++11
+
+// <barrier>
+
+#include <barrier>
+#include <thread>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main(int, char**)
+{
+  int x = 0;
+  auto comp = [&]() { x += 1; };
+  std::barrier<decltype(comp)> b(2, comp);
+
+  std::thread t([&](){
+      for(int i = 0; i < 10; ++i)
+        b.arrive_and_wait();
+  });
+
+  for(int i = 0; i < 10; ++i)
+    b.arrive_and_wait();
+
+  assert(x == 10);
+  t.join();
+  return 0;
+}