diff --git a/libcxx/include/__algorithm/copy_backward.h b/libcxx/include/__algorithm/copy_backward.h
index 02ffc14361e6a..9f890645a41d3 100644
--- a/libcxx/include/__algorithm/copy_backward.h
+++ b/libcxx/include/__algorithm/copy_backward.h
@@ -52,7 +52,7 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI __bit_iterator<_Cp, false> _
       difference_type __dn = std::min(static_cast<difference_type>(__last.__ctz_), __n);
       __n -= __dn;
       unsigned __clz     = __bits_per_word - __last.__ctz_;
-      __storage_type __m = (~__storage_type(0) << (__last.__ctz_ - __dn)) & (~__storage_type(0) >> __clz);
+      __storage_type __m = std::__middle_mask<__storage_type>(__clz, __last.__ctz_ - __dn);
       __storage_type __b = *__last.__seg_ & __m;
       *__result.__seg_ &= ~__m;
       *__result.__seg_ |= __b;
@@ -69,7 +69,7 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI __bit_iterator<_Cp, false> _
     __n -= __nw * __bits_per_word;
     // do last word
     if (__n > 0) {
-      __storage_type __m = ~__storage_type(0) << (__bits_per_word - __n);
+      __storage_type __m = std::__leading_mask<__storage_type>(__bits_per_word - __n);
       __storage_type __b = *--__last.__seg_ & __m;
       *--__result.__seg_ &= ~__m;
       *__result.__seg_ |= __b;
@@ -94,12 +94,12 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI __bit_iterator<_Cp, false> _
       difference_type __dn = std::min(static_cast<difference_type>(__last.__ctz_), __n);
       __n -= __dn;
       unsigned __clz_l     = __bits_per_word - __last.__ctz_;
-      __storage_type __m   = (~__storage_type(0) << (__last.__ctz_ - __dn)) & (~__storage_type(0) >> __clz_l);
+      __storage_type __m   = std::__middle_mask<__storage_type>(__clz_l, __last.__ctz_ - __dn);
       __storage_type __b   = *__last.__seg_ & __m;
       unsigned __clz_r     = __bits_per_word - __result.__ctz_;
       __storage_type __ddn = std::min(__dn, static_cast<difference_type>(__result.__ctz_));
       if (__ddn > 0) {
-        __m = (~__storage_type(0) << (__result.__ctz_ - __ddn)) & (~__storage_type(0) >> __clz_r);
+        __m = std::__middle_mask<__storage_type>(__clz_r, __result.__ctz_ - __ddn);
         *__result.__seg_ &= ~__m;
         if (__result.__ctz_ > __last.__ctz_)
           *__result.__seg_ |= __b << (__result.__ctz_ - __last.__ctz_);
@@ -112,7 +112,7 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI __bit_iterator<_Cp, false> _
         // __result.__ctz_ == 0
         --__result.__seg_;
         __result.__ctz_ = static_cast<unsigned>(-__dn & (__bits_per_word - 1));
-        __m             = ~__storage_type(0) << __result.__ctz_;
+        __m             = std::__leading_mask<__storage_type>(__result.__ctz_);
         *__result.__seg_ &= ~__m;
         __last.__ctz_ -= __dn + __ddn;
         *__result.__seg_ |= __b << (__result.__ctz_ - __last.__ctz_);
@@ -123,7 +123,7 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI __bit_iterator<_Cp, false> _
     // __result.__ctz_ != 0 || __n == 0
     // do middle words
     unsigned __clz_r   = __bits_per_word - __result.__ctz_;
-    __storage_type __m = ~__storage_type(0) >> __clz_r;
+    __storage_type __m = std::__trailing_mask<__storage_type>(__clz_r);
     for (; __n >= __bits_per_word; __n -= __bits_per_word) {
       __storage_type __b = *--__last.__seg_;
       *__result.__seg_ &= ~__m;
@@ -133,11 +133,11 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI __bit_iterator<_Cp, false> _
     }
     // do last word
     if (__n > 0) {
-      __m                 = ~__storage_type(0) << (__bits_per_word - __n);
+      __m                 = std::__leading_mask<__storage_type>(__bits_per_word - __n);
       __storage_type __b  = *--__last.__seg_ & __m;
       __clz_r             = __bits_per_word - __result.__ctz_;
       __storage_type __dn = std::min(__n, static_cast<difference_type>(__result.__ctz_));
-      __m                 = (~__storage_type(0) << (__result.__ctz_ - __dn)) & (~__storage_type(0) >> __clz_r);
+      __m                 = std::__middle_mask<__storage_type>(__clz_r, __result.__ctz_ - __dn);
       *__result.__seg_ &= ~__m;
       *__result.__seg_ |= __b >> (__bits_per_word - __result.__ctz_);
       __result.__ctz_ = static_cast<unsigned>(((-__dn & (__bits_per_word - 1)) + __result.__ctz_) % __bits_per_word);
@@ -146,7 +146,7 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI __bit_iterator<_Cp, false> _
         // __result.__ctz_ == 0
         --__result.__seg_;
         __result.__ctz_ = static_cast<unsigned>(-__n & (__bits_per_word - 1));
-        __m             = ~__storage_type(0) << __result.__ctz_;
+        __m             = std::__leading_mask<__storage_type>(__result.__ctz_);
         *__result.__seg_ &= ~__m;
         *__result.__seg_ |= __b << (__result.__ctz_ - (__bits_per_word - __n - __dn));
       }
diff --git a/libcxx/include/__bit_reference b/libcxx/include/__bit_reference
index 58cbd4d51c88e..f5c22fc0a3ade 100644
--- a/libcxx/include/__bit_reference
+++ b/libcxx/include/__bit_reference
@@ -82,13 +82,20 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _StorageType __trailing_mask
   return static_cast<_StorageType>(~static_cast<_StorageType>(0)) >> __clz;
 }
 
+// Creates a mask of type `_StorageType` with a specified number of trailing zeros (__ctz) and sets all remaining
+// bits to one.
+template <class _StorageType>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _StorageType __leading_mask(unsigned __ctz) {
+  static_assert(is_unsigned<_StorageType>::value, "__leading_mask only works with unsigned types");
+  return static_cast<_StorageType>(~static_cast<_StorageType>(0)) << __ctz;
+}
+
 // Creates a mask of type `_StorageType` with a specified number of leading zeros (__clz), a specified number of
 // trailing zeros (__ctz), and sets all bits in between to one.
 template <class _StorageType>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _StorageType __middle_mask(unsigned __clz, unsigned __ctz) {
   static_assert(is_unsigned<_StorageType>::value, "__middle_mask only works with unsigned types");
-  return (static_cast<_StorageType>(~static_cast<_StorageType>(0)) << __ctz) &
-         std::__trailing_mask<_StorageType>(__clz);
+  return std::__leading_mask<_StorageType>(__ctz) & std::__trailing_mask<_StorageType>(__clz);
 }
 
 // This function is designed to operate correctly even for smaller integral types like `uint8_t`, `uint16_t`,
diff --git a/libcxx/include/__fwd/bit_reference.h b/libcxx/include/__fwd/bit_reference.h
index e212667f3de1f..99d1fdbc41959 100644
--- a/libcxx/include/__fwd/bit_reference.h
+++ b/libcxx/include/__fwd/bit_reference.h
@@ -33,6 +33,9 @@ __fill_masked_range(_StoragePointer __word, unsigned __ctz, unsigned __clz, bool
 template <class _StorageType>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _StorageType __trailing_mask(unsigned __clz);
 
+template <class _StorageType>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _StorageType __leading_mask(unsigned __ctz);
+
 template <class _StorageType>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _StorageType __middle_mask(unsigned __clz, unsigned __ctz);
 
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/copy_backward.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/copy_backward.pass.cpp
index 8a528a96f5294..def192d4d6637 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/copy_backward.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/copy_backward.pass.cpp
@@ -13,10 +13,13 @@
 //   constexpr OutIter   // constexpr after C++17
 //   copy_backward(InIter first, InIter last, OutIter result);
 
+// XFAIL: FROZEN-CXX03-HEADERS-FIXME
+
 #include <algorithm>
 #include <cassert>
 #include <vector>
 
+#include "sized_allocator.h"
 #include "test_macros.h"
 #include "test_iterators.h"
 #include "type_algorithms.h"
@@ -111,6 +114,146 @@ TEST_CONSTEXPR_CXX20 bool test() {
     assert(test_vector_bool(256));
   }
 
+  // Validate std::copy_backward with std::vector<bool> iterators and custom storage types.
+  // Ensure that assigned bits hold the intended values, while unassigned bits stay unchanged.
+  // Related issue: https://github.com/llvm/llvm-project/issues/131718.
+  {
+    //// Tests for std::copy_backward with aligned bits
+
+    { // Test the first (partial) word for uint8_t
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(7, false, Alloc(1));
+      std::vector<bool, Alloc> out(8, true, Alloc(1));
+      std::copy_backward(in.begin(), in.begin() + 1, out.begin() + 1);
+      assert(out[0] == false);
+      for (std::size_t i = 1; i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+    { // Test the last (partial) word for uint8_t
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(8, false, Alloc(1));
+      for (std::size_t i = 0; i < in.size(); i += 2)
+        in[i] = true;
+      std::vector<bool, Alloc> out(8, true, Alloc(1));
+      std::copy_backward(in.end() - 4, in.end(), out.end());
+      for (std::size_t i = 0; i < static_cast<std::size_t>(in.size() - 4); ++i)
+        assert(out[i] == true);
+      for (std::size_t i = in.size() + 4; i < out.size(); ++i)
+        assert(in[i] == out[i]);
+    }
+    { // Test the middle (whole) words for uint8_t
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(17, false, Alloc(1));
+      for (std::size_t i = 0; i < in.size(); i += 2)
+        in[i] = true;
+      std::vector<bool, Alloc> out(24, true, Alloc(1));
+      std::copy_backward(in.begin(), in.end(), out.begin() + in.size());
+      for (std::size_t i = 0; i < in.size(); ++i)
+        assert(in[i] == out[i]);
+      for (std::size_t i = in.size(); i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+
+    { // Test the first (partial) word for uint16_t
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(14, false, Alloc(1));
+      std::vector<bool, Alloc> out(16, true, Alloc(1));
+      std::copy_backward(in.begin(), in.begin() + 2, out.begin() + 2);
+      assert(out[0] == false);
+      assert(out[1] == false);
+      for (std::size_t i = 2; i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+    { // Test the last (partial) word for uint16_t
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(16, false, Alloc(1));
+      for (std::size_t i = 0; i < in.size(); i += 2)
+        in[i] = true;
+      std::vector<bool, Alloc> out(16, true, Alloc(1));
+      std::copy_backward(in.end() - 8, in.end(), out.end());
+      for (std::size_t i = 0; i < static_cast<std::size_t>(in.size() - 8); ++i)
+        assert(out[i] == true);
+      for (std::size_t i = in.size() + 8; i < out.size(); ++i)
+        assert(in[i] == out[i]);
+    }
+    { // Test the middle (whole) words for uint16_t
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(34, false, Alloc(1));
+      for (std::size_t i = 0; i < in.size(); i += 2)
+        in[i] = true;
+      std::vector<bool, Alloc> out(48, true, Alloc(1));
+      std::copy_backward(in.begin(), in.end(), out.begin() + in.size());
+      for (std::size_t i = 0; i < in.size(); ++i)
+        assert(in[i] == out[i]);
+      for (std::size_t i = in.size(); i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+
+    //// Tests for std::copy_backward with unaligned bits
+
+    { // Test the first (partial) word for uint8_t
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(8, false, Alloc(1));
+      std::vector<bool, Alloc> out(8, true, Alloc(1));
+      std::copy_backward(in.begin(), in.begin() + 1, out.begin() + 1);
+      assert(out[0] == false);
+      for (std::size_t i = 1; i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+    { // Test the last (partial) word for uint8_t
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(8, false, Alloc(1));
+      std::vector<bool, Alloc> out(8, true, Alloc(1));
+      std::copy_backward(in.end() - 1, in.end(), out.begin() + 1);
+      assert(out[0] == false);
+      for (std::size_t i = 1; i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+    { // Test the middle (whole) words for uint8_t
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(16, false, Alloc(1));
+      for (std::size_t i = 0; i < in.size(); i += 2)
+        in[i] = true;
+      std::vector<bool, Alloc> out(17, true, Alloc(1));
+      std::copy_backward(in.begin(), in.end(), out.end());
+      assert(out[0] == true);
+      for (std::size_t i = 0; i < in.size(); ++i)
+        assert(in[i] == out[i + 1]);
+    }
+
+    { // Test the first (partial) word for uint16_t
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(16, false, Alloc(1));
+      std::vector<bool, Alloc> out(16, true, Alloc(1));
+      std::copy_backward(in.begin(), in.begin() + 2, out.begin() + 2);
+      assert(out[0] == false);
+      assert(out[1] == false);
+      for (std::size_t i = 2; i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+    { // Test the last (partial) word for uint16_t
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(16, false, Alloc(1));
+      std::vector<bool, Alloc> out(16, true, Alloc(1));
+      std::copy_backward(in.end() - 2, in.end(), out.begin() + 2);
+      assert(out[0] == false);
+      assert(out[1] == false);
+      for (std::size_t i = 2; i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+    { // Test the middle (whole) words for uint16_t
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(32, false, Alloc(1));
+      for (std::size_t i = 0; i < in.size(); i += 2)
+        in[i] = true;
+      std::vector<bool, Alloc> out(33, true, Alloc(1));
+      std::copy_backward(in.begin(), in.end(), out.end());
+      assert(out[0] == true);
+      for (std::size_t i = 0; i < in.size(); ++i)
+        assert(in[i] == out[i + 1]);
+    }
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/ranges.copy_backward.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/ranges.copy_backward.pass.cpp
index a7fa3db23e6ba..e7251ab905db5 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/ranges.copy_backward.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/ranges.copy_backward.pass.cpp
@@ -28,6 +28,7 @@
 #include <vector>
 
 #include "almost_satisfies_types.h"
+#include "sized_allocator.h"
 #include "test_iterators.h"
 #include "test_macros.h"
 
@@ -355,6 +356,146 @@ constexpr bool test() {
     assert(test_vector_bool(199));
     assert(test_vector_bool(256));
   }
+
+  // Validate std::ranges::copy_backward with std::vector<bool> iterators and custom storage types.
+  // Ensure that assigned bits hold the intended values, while unassigned bits stay unchanged.
+  // Related issue: https://github.com/llvm/llvm-project/issues/131718.
+  {
+    //// Tests for std::ranges::copy_backward with aligned bits
+
+    { // Test the first (partial) word for uint8_t
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(7, false, Alloc(1));
+      std::vector<bool, Alloc> out(8, true, Alloc(1));
+      std::ranges::copy_backward(std::ranges::subrange(in.begin(), in.begin() + 1), out.begin() + 1);
+      assert(out[0] == false);
+      for (std::size_t i = 1; i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+    { // Test the last (partial) word for uint8_t
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(8, false, Alloc(1));
+      for (std::size_t i = 0; i < in.size(); i += 2)
+        in[i] = true;
+      std::vector<bool, Alloc> out(8, true, Alloc(1));
+      std::ranges::copy_backward(std::ranges::subrange(in.end() - 4, in.end()), out.end());
+      for (std::size_t i = 0; i < static_cast<std::size_t>(in.size() - 4); ++i)
+        assert(out[i] == true);
+      for (std::size_t i = in.size() + 4; i < out.size(); ++i)
+        assert(in[i] == out[i]);
+    }
+    { // Test the middle (whole) words for uint8_t
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(17, false, Alloc(1));
+      for (std::size_t i = 0; i < in.size(); i += 2)
+        in[i] = true;
+      std::vector<bool, Alloc> out(24, true, Alloc(1));
+      std::ranges::copy_backward(std::ranges::subrange(in.begin(), in.end()), out.begin() + in.size());
+      for (std::size_t i = 0; i < in.size(); ++i)
+        assert(in[i] == out[i]);
+      for (std::size_t i = in.size(); i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+
+    { // Test the first (partial) word for uint16_t
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(14, false, Alloc(1));
+      std::vector<bool, Alloc> out(16, true, Alloc(1));
+      std::ranges::copy_backward(std::ranges::subrange(in.begin(), in.begin() + 2), out.begin() + 2);
+      assert(out[0] == false);
+      assert(out[1] == false);
+      for (std::size_t i = 2; i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+    { // Test the last (partial) word for uint16_t
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(16, false, Alloc(1));
+      for (std::size_t i = 0; i < in.size(); i += 2)
+        in[i] = true;
+      std::vector<bool, Alloc> out(16, true, Alloc(1));
+      std::ranges::copy_backward(std::ranges::subrange(in.end() - 8, in.end()), out.end());
+      for (std::size_t i = 0; i < static_cast<std::size_t>(in.size() - 8); ++i)
+        assert(out[i] == true);
+      for (std::size_t i = in.size() + 8; i < out.size(); ++i)
+        assert(in[i] == out[i]);
+    }
+    { // Test the middle (whole) words for uint16_t
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(34, false, Alloc(1));
+      for (std::size_t i = 0; i < in.size(); i += 2)
+        in[i] = true;
+      std::vector<bool, Alloc> out(48, true, Alloc(1));
+      std::ranges::copy_backward(std::ranges::subrange(in.begin(), in.end()), out.begin() + in.size());
+      for (std::size_t i = 0; i < in.size(); ++i)
+        assert(in[i] == out[i]);
+      for (std::size_t i = in.size(); i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+
+    //// Tests for std::ranges::copy_backward with unaligned bits
+
+    { // Test the first (partial) word for uint8_t
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(8, false, Alloc(1));
+      std::vector<bool, Alloc> out(8, true, Alloc(1));
+      std::ranges::copy_backward(std::ranges::subrange(in.begin(), in.begin() + 1), out.begin() + 1);
+      assert(out[0] == false);
+      for (std::size_t i = 1; i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+    { // Test the last (partial) word for uint8_t
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(8, false, Alloc(1));
+      std::vector<bool, Alloc> out(8, true, Alloc(1));
+      std::ranges::copy_backward(std::ranges::subrange(in.end() - 1, in.end()), out.begin() + 1);
+      assert(out[0] == false);
+      for (std::size_t i = 1; i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+    { // Test the middle (whole) words for uint8_t
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(16, false, Alloc(1));
+      for (std::size_t i = 0; i < in.size(); i += 2)
+        in[i] = true;
+      std::vector<bool, Alloc> out(17, true, Alloc(1));
+      std::ranges::copy_backward(std::ranges::subrange(in.begin(), in.end()), out.end());
+      assert(out[0] == true);
+      for (std::size_t i = 0; i < in.size(); ++i)
+        assert(in[i] == out[i + 1]);
+    }
+
+    { // Test the first (partial) word for uint16_t
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(16, false, Alloc(1));
+      std::vector<bool, Alloc> out(16, true, Alloc(1));
+      std::ranges::copy_backward(std::ranges::subrange(in.begin(), in.begin() + 2), out.begin() + 2);
+      assert(out[0] == false);
+      assert(out[1] == false);
+      for (std::size_t i = 2; i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+    { // Test the last (partial) word for uint16_t
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(16, false, Alloc(1));
+      std::vector<bool, Alloc> out(16, true, Alloc(1));
+      std::ranges::copy_backward(std::ranges::subrange(in.end() - 2, in.end()), out.begin() + 2);
+      assert(out[0] == false);
+      assert(out[1] == false);
+      for (std::size_t i = 2; i < out.size(); ++i)
+        assert(out[i] == true);
+    }
+    { // Test the middle (whole) words for uint16_t
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(32, false, Alloc(1));
+      for (std::size_t i = 0; i < in.size(); i += 2)
+        in[i] = true;
+      std::vector<bool, Alloc> out(33, true, Alloc(1));
+      std::ranges::copy_backward(std::ranges::subrange(in.begin(), in.end()), out.end());
+      assert(out[0] == true);
+      for (std::size_t i = 0; i < in.size(); ++i)
+        assert(in[i] == out[i + 1]);
+    }
+  }
 #endif
 
   return true;