Index: libcxx/include/barrier
===================================================================
--- libcxx/include/barrier
+++ libcxx/include/barrier
@@ -300,6 +300,13 @@
     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
     barrier(ptrdiff_t __count, _CompletionF __completion = _CompletionF())
         : __b_(__count, _VSTD::move(__completion)) {
+        _LIBCPP_ASSERT_UNCATEGORIZED(
+            __count >= 0,
+            "barrier::barrier(ptrdiff_t, CompletionFunction): barrier cannot be initialized with a negative value");
+        _LIBCPP_ASSERT_UNCATEGORIZED(
+            __count <= max(),
+            "barrier::barrier(ptrdiff_t, CompletionFunction): barrier cannot be initialized with "
+            "a value greater than max()");
     }
 
     barrier(barrier const&) = delete;
@@ -308,6 +315,7 @@
     [[__nodiscard__]] _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
     arrival_token arrive(ptrdiff_t __update = 1)
     {
+        _LIBCPP_ASSERT_UNCATEGORIZED(__update > 0, "barrier:arrive must be called with a value greater than 0");
         return __b_.arrive(__update);
     }
     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
Index: libcxx/test/libcxx/thread/thread.barrier/assert.arrive.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/libcxx/thread/thread.barrier/assert.arrive.pass.cpp
@@ -0,0 +1,34 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <barrier>
+
+// class barrier;
+
+// void arrive(ptrdiff_t __update = 1);
+
+// Make sure that calling arrive with a negative value triggers an assertion.
+
+// REQUIRES: has-unix-headers
+// XFAIL: availability-verbose_abort-missing
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+#include <barrier>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  {
+    std::barrier<> b(5);
+    TEST_LIBCPP_ASSERT_FAILURE(b.arrive(0), "barrier:arrive must be called with a value greater than 0");
+  }
+
+  return 0;
+}
Index: libcxx/test/libcxx/thread/thread.barrier/assert.ctor.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/libcxx/thread/thread.barrier/assert.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, c++14, c++17
+
+// <barrier>
+
+// class barrier;
+
+// barrier(ptrdiff_t __count, _CompletionF __completion = _CompletionF());
+
+// Make sure that constructing barrier with a negative value triggers an assertion
+
+// REQUIRES: has-unix-headers
+// XFAIL: availability-verbose_abort-missing
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+#include <barrier>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  {
+    TEST_LIBCPP_ASSERT_FAILURE(
+        [] { std::barrier<> b(-1); }(),
+        "barrier::barrier(ptrdiff_t, CompletionFunction): barrier cannot be initialized with a negative value");
+  }
+
+  // We can't check the precondition for max() because there's no value
+  // that would violate the precondition (in our implementation)
+
+  return 0;
+}