diff --git a/libcxx/include/__flat_set/utils.h b/libcxx/include/__flat_set/utils.h
index ed3b4c48580fb..542bfd886aef5 100644
--- a/libcxx/include/__flat_set/utils.h
+++ b/libcxx/include/__flat_set/utils.h
@@ -11,6 +11,7 @@
 #define _LIBCPP___FLAT_SET_UTILS_H
 
 #include <__config>
+#include <__iterator/iterator_traits.h>
 #include <__ranges/access.h>
 #include <__ranges/concepts.h>
 #include <__type_traits/container_traits.h>
@@ -60,7 +61,8 @@ struct __flat_set_utils {
       // C++23 Sequence Container should have insert_range member function
       // Note that not all Sequence Containers provide append_range.
       __set.__keys_.insert_range(__set.__keys_.end(), std::forward<_Range>(__rng));
-    } else if constexpr (ranges::common_range<_Range>) {
+    } else if constexpr (ranges::common_range<_Range> &&
+                         __has_input_iterator_category<ranges::iterator_t<_Range>>::value) {
       __set.__keys_.insert(__set.__keys_.end(), ranges::begin(__rng), ranges::end(__rng));
     } else {
       for (auto&& __x : __rng) {
diff --git a/libcxx/test/libcxx/containers/container.adaptors/flat.multiset/insert_range.pass.cpp b/libcxx/test/libcxx/containers/container.adaptors/flat.multiset/insert_range.pass.cpp
new file mode 100644
index 0000000000000..3ca021c2ac650
--- /dev/null
+++ b/libcxx/test/libcxx/containers/container.adaptors/flat.multiset/insert_range.pass.cpp
@@ -0,0 +1,43 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: c++03, c++11, c++14, c++17, c++20
+// UNSUPPORTED: no-localization
+
+// <flat_set>
+
+// As an extension, libc++ flat containers support inserting a non forward range into
+// a pre-C++23 container that doesn't provide insert_range(...), since many containers
+// out there are in that situation.
+// https://github.com/llvm/llvm-project/issues/136656
+
+#include <flat_set>
+#include <ranges>
+#include <sstream>
+#include <vector>
+
+#include "MinSequenceContainer.h"
+#include "test_macros.h"
+
+void test() {
+  MinSequenceContainer<int> v;
+  std::flat_multiset s(v);
+  std::istringstream ints("0 1 1 0");
+  auto r = std::ranges::subrange(std::istream_iterator<int>(ints), std::istream_iterator<int>()) |
+           std::views::transform([](int i) { return i * i; });
+  static_assert(
+      ![](auto& t) { return requires { t.insert_range(r); }; }(v),
+      "This test is to test the case where the underlying container does not provide insert_range");
+  s.insert_range(r);
+}
+
+int main(int, char**) {
+  test();
+
+  return 0;
+}
diff --git a/libcxx/test/libcxx/containers/container.adaptors/flat.set/insert_range.pass.cpp b/libcxx/test/libcxx/containers/container.adaptors/flat.set/insert_range.pass.cpp
new file mode 100644
index 0000000000000..8023e251ccb17
--- /dev/null
+++ b/libcxx/test/libcxx/containers/container.adaptors/flat.set/insert_range.pass.cpp
@@ -0,0 +1,43 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: c++03, c++11, c++14, c++17, c++20
+// UNSUPPORTED: no-localization
+
+// <flat_set>
+
+// As an extension, libc++ flat containers support inserting a non forward range into
+// a pre-C++23 container that doesn't provide insert_range(...), since many containers
+// out there are in that situation.
+// https://github.com/llvm/llvm-project/issues/136656
+
+#include <flat_set>
+#include <ranges>
+#include <sstream>
+#include <vector>
+
+#include "MinSequenceContainer.h"
+#include "test_macros.h"
+
+void test() {
+  MinSequenceContainer<int> v;
+  std::flat_set s(v);
+  std::istringstream ints("0 1 1 0");
+  auto r = std::ranges::subrange(std::istream_iterator<int>(ints), std::istream_iterator<int>()) |
+           std::views::transform([](int i) { return i * i; });
+  static_assert(
+      ![](auto& t) { return requires { t.insert_range(r); }; }(v),
+      "This test is to test the case where the underlying container does not provide insert_range");
+  s.insert_range(r);
+}
+
+int main(int, char**) {
+  test();
+
+  return 0;
+}