Index: libcxx/include/barrier
===================================================================
--- libcxx/include/barrier
+++ libcxx/include/barrier
@@ -298,7 +298,7 @@
     }
 
     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
-    barrier(ptrdiff_t __count, _CompletionF __completion = _CompletionF())
+    constexpr explicit barrier(ptrdiff_t __count, _CompletionF __completion = _CompletionF())
         : __b_(__count, _VSTD::move(__completion)) {
     }
 
Index: libcxx/test/std/thread/thread.barrier/ctor.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/thread/thread.barrier/ctor.pass.cpp
@@ -0,0 +1,42 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: no-threads
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// XFAIL: availability-synchronization_library-missing
+
+// <barrier>
+
+// constexpr explicit barrier(ptrdiff_t __count, _CompletionF __completion = _CompletionF());
+
+// Make sure that the ctor of barrier is constexpr and explicit.
+
+#include <barrier>
+
+#if TEST_STD_VER >= 11
+#  include "test_convertible.h"
+#endif
+#include "test_macros.h"
+
+TEST_CONSTEXPR_CXX20 bool test() { std::barrier<> b(2); }
+
+int main(int, char**) {
+  test();
+
+#if TEST_STD_VER > 17
+  static_assert(test());
+#endif
+
+#if TEST_STD_VER >= 11
+  typedef std::barrier<> B;
+  static_assert(!test_convertible<std::barrier, std::ptrdiff_t, std::_CompletionF>(), "This constructor must be explicit");
+#endif
+
+  return 0;
+}