Index: libcxx/include/barrier
===================================================================
--- libcxx/include/barrier
+++ libcxx/include/barrier
@@ -129,10 +129,10 @@
     }
 
     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
-    __barrier_base(ptrdiff_t __expected, _CompletionF __completion = _CompletionF())
-            : __expected_(__expected), __base_(std::__construct_barrier_algorithm_base(this->__expected_),
-                                               &__destroy_barrier_algorithm_base),
-              __expected_adjustment_(0), __completion_(std::move(__completion)), __phase_(0)
+    constexpr explicit __barrier_base(ptrdiff_t __expected, _CompletionF __completion = _CompletionF())
+        : __expected_(__expected), __base_(std::__construct_barrier_algorithm_base(this->__expected_),
+                                           &__destroy_barrier_algorithm_base),
+        __expected_adjustment_(0), __completion_(std::move(__completion)), __phase_(0)
     {
     }
     [[__nodiscard__]] _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
@@ -195,7 +195,7 @@
     }
 
     _LIBCPP_INLINE_VISIBILITY
-    __barrier_base(ptrdiff_t __expected, _CompletionF __completion = _CompletionF())
+    constexpr explicit __barrier_base(ptrdiff_t __expected, _CompletionF __completion = _CompletionF())
         : __expected(__expected), __arrived(__expected), __completion(std::move(__completion)), __phase(false)
     {
     }
@@ -252,7 +252,7 @@
     }
 
     _LIBCPP_INLINE_VISIBILITY
-    explicit inline __barrier_base(ptrdiff_t __count, __empty_completion = __empty_completion())
+    constexpr explicit __barrier_base(ptrdiff_t __count, __empty_completion = __empty_completion())
         : __phase_arrived_expected(__init(__count))
     {
     }
@@ -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 +1,38 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// 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>
+
+#include "test_convertible.h"
+
+static_assert(!test_convertible<std::barrier<>, std::ptrdiff_t>(), "This constructor must be explicit");
+
+constexpr bool test() {
+  constexpr auto comp = []() noexcept {};
+  [[maybe_unused]] std::barrier<> a{5};
+  [[maybe_unused]] std::barrier<decltype(comp)> b{5, comp};
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}