Index: libcxx/include/semaphore
===================================================================
--- libcxx/include/semaphore
+++ libcxx/include/semaphore
@@ -81,6 +81,8 @@
 
 */
 
+#define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
+
 class __atomic_semaphore_base
 {
     __atomic_base<ptrdiff_t> __a_;
@@ -93,9 +95,12 @@
     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
     void release(ptrdiff_t __update = 1)
     {
-        if(0 < __a_.fetch_add(__update, memory_order_release))
+        auto __old = __a_.fetch_add(__update, memory_order_release);
+        _LIBCPP_ASSERT_UNCATEGORIZED(__update <= _LIBCPP_SEMAPHORE_MAX - __old, "update is greater than the expected value");
+
+        if (0 < __old)
             ;
-        else if(__update > 1)
+        else if (__update > 1)
             __a_.notify_all();
         else
             __a_.notify_one();
@@ -131,8 +136,6 @@
     }
 };
 
-#define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
-
 template<ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
 class counting_semaphore
 {
@@ -144,7 +147,17 @@
     }
 
     _LIBCPP_INLINE_VISIBILITY
-    constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore_(__count) { }
+    constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore_(__count)
+    {
+        _LIBCPP_ASSERT_UNCATEGORIZED(
+            __count >= 0,
+            "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
+            "initialized with a negative value");
+        _LIBCPP_ASSERT_UNCATEGORIZED(
+            __count <= max(),
+            "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
+            "initialized with a value greater than max()");
+    }
     ~counting_semaphore() = default;
 
     counting_semaphore(const counting_semaphore&) = delete;
@@ -153,6 +166,7 @@
     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
     void release(ptrdiff_t __update = 1)
     {
+        _LIBCPP_ASSERT_UNCATEGORIZED(__update >= 0, "counting_semaphore:release called with a negative value");
         __semaphore_.release(__update);
     }
     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
Index: libcxx/test/libcxx/thread/thread.semaphore/assert.ctor.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/libcxx/thread/thread.semaphore/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
+
+// REQUIRES: availability-synchronization_library-missing
+// XFAIL: target={{.+}}-apple-macosx10.{{13|15}}
+
+// REQUIRES: has-unix-headers
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+// <semaphore>
+
+// constexpr explicit counting_semaphore(ptrdiff_t __count);
+
+// Make sure that constructing counting_semaphore with a negative value triggers an assertion
+
+#include <semaphore>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  {
+    TEST_LIBCPP_ASSERT_FAILURE(
+        [] { std::counting_semaphore<> s(-1); }(),
+        "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore 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;
+}
Index: libcxx/test/libcxx/thread/thread.semaphore/assert.release.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/libcxx/thread/thread.semaphore/assert.release.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
+
+// REQUIRES: availability-synchronization_library-missing
+// XFAIL: target={{.+}}-apple-macosx10.{{13|15}}
+
+// REQUIRES: has-unix-headers
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+// <semaphore>
+
+// void release(ptrdiff_t __update = 1);
+
+// Make sure that calling release with a negative value triggers or with a value
+// greater than expected triggers an assertion
+
+#include <semaphore>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  {
+    std::counting_semaphore<> s(2);
+    TEST_LIBCPP_ASSERT_FAILURE(s.release(-1), "counting_semaphore:release called with a negative value");
+  }
+
+  {
+    // Call release with an arbitrary larger than expected value
+    std::counting_semaphore<> s(2);
+    TEST_LIBCPP_ASSERT_FAILURE(
+        s.release(std::counting_semaphore<>::max()), "update is greater than the expected value");
+  }
+
+  return 0;
+}