diff --git a/libcxx/docs/Status/Cxx2bIssues.csv b/libcxx/docs/Status/Cxx2bIssues.csv
--- a/libcxx/docs/Status/Cxx2bIssues.csv
+++ b/libcxx/docs/Status/Cxx2bIssues.csv
@@ -97,3 +97,4 @@
 `3553 <https://wg21.link/LWG3553>`__,"Useless constraint in ``split_view::outer-iterator::value_type::begin()``","June 2021","",""
 `3555 <https://wg21.link/LWG3555>`__,"``{transform,elements}_view::iterator::iterator_concept`` should consider const-qualification of the underlying range","June 2021","",""
 "","","","",""
+`3343 <https://wg21.link/LWG3343>`__,"Ordering of calls to ``unlock()`` and ``notify_all()`` in Effects element of ``notify_all_at_thread_exit()`` should be reversed","Not Yet Adopted","|Complete|","14.0"
diff --git a/libcxx/src/thread.cpp b/libcxx/src/thread.cpp
--- a/libcxx/src/thread.cpp
+++ b/libcxx/src/thread.cpp
@@ -158,8 +158,8 @@
     for (_Notify::iterator i = notify_.begin(), e = notify_.end();
             i != e; ++i)
     {
-        i->second->unlock();
         i->first->notify_all();
+        i->second->unlock();
     }
     for (_AsyncStates::iterator i = async_states_.begin(), e = async_states_.end();
             i != e; ++i)
diff --git a/libcxx/test/std/thread/thread.condition/notify_all_at_thread_exit_lwg3343.pass.cpp b/libcxx/test/std/thread/thread.condition/notify_all_at_thread_exit_lwg3343.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/thread/thread.condition/notify_all_at_thread_exit_lwg3343.pass.cpp
@@ -0,0 +1,94 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// notify_all_at_thread_exit(...) requires move semantics to transfer the
+// unique_lock.
+// UNSUPPORTED: c++03
+
+// <condition_variable>
+
+// void
+//   notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);
+
+#include "make_test_thread.h"
+#include "test_macros.h"
+
+#include <condition_variable>
+#include <cassert>
+#include <chrono>
+#include <memory>
+#include <mutex>
+#include <thread>
+
+union X {
+    X() : cv_() {}
+    ~X() {}
+    std::condition_variable cv_;
+    unsigned char bytes_[sizeof(std::condition_variable)];
+};
+
+void test()
+{
+    constexpr int N = 3;
+
+    X x;
+    std::mutex m;
+    int threads_active = N;
+
+    for (int i = 0; i < N; ++i) {
+        std::thread t = support::make_test_thread([&] {
+            // Signal thread completion
+            std::unique_lock<std::mutex> lk(m);
+            --threads_active;
+            std::notify_all_at_thread_exit(x.cv_, std::move(lk));
+            std::this_thread::sleep_for(std::chrono::milliseconds(1));
+        });
+        t.detach();
+    }
+
+    // Wait until all threads complete, i.e. until they've all
+    // decremented `threads_active` and then unlocked `m` at thread exit.
+    // It is possible that this `wait` may spuriously wake up,
+    // but it won't be able to continue until the last thread
+    // unlocks `m`.
+    {
+        std::unique_lock<std::mutex> lk(m);
+        x.cv_.wait(lk, [&]() { return threads_active == 0; });
+    }
+
+    // Destroy the condition_variable and shred the bytes.
+    // Simulate reusing the memory for something else.
+    x.cv_.~condition_variable();
+    for (unsigned char& c : x.bytes_) {
+        c = 0xcd;
+    }
+
+    DoNotOptimize(x.bytes_);
+
+    // Check that the bytes still have the same value we just wrote to them.
+    // If any thread wrongly unlocked `m` before calling cv.notify_all(), and
+    // cv.notify_all() writes to the memory of the cv, then we have a chance
+    // to detect the problem here.
+    int sum = 0;
+    for (unsigned char c : x.bytes_) {
+       sum += c;
+    }
+    DoNotOptimize(sum);
+    assert(sum == (0xcd * sizeof(std::condition_variable)));
+}
+
+int main(int, char**)
+{
+    for (int i = 0; i < 10000; ++i) {
+        test();
+    }
+
+    return 0;
+}