[libc++][test] Augment ranges::{fill, fill_n, find} with missing tests (#121209)

libc++ currently has very limited test coverage for `std::ranges{fill, fill_n, find}`
with `vector<bool>::iterator` optimizations. Specifically, the existing tests for
`std::ranges::fill` only covers cases of 1 - 2 bytes, which is merely 1/8 to 1/4
of the `__storage_type` word size. This renders the tests insufficient to validate
functionality for whole words, with or without partial words (which necessitates at
least 8 bytes of data). Moreover, no tests were provided for `ranges::{find, fill_n}`
with `vector<bool>::iterator` optimizations. This PR fills in the gap.

6 files changed, 371 insertions, 175 deletions

diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill.pass.cpp
index 0e532ae..9b403db 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill.pass.cpp
@@ -48,89 +48,96 @@ struct Test {
   }
 };
 
-// Make sure std::fill behaves properly with std::vector<bool> iterators with custom size types.
-// See https://github.com/llvm/llvm-project/pull/122410.
-TEST_CONSTEXPR_CXX20 void test_bititer_with_custom_sized_types() {
-  {
-    using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
-    std::vector<bool, Alloc> in(100, false, Alloc(1));
-    std::vector<bool, Alloc> expected(100, true, Alloc(1));
-    std::fill(in.begin(), in.end(), true);
-    assert(in == expected);
-  }
-  {
-    using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
-    std::vector<bool, Alloc> in(200, false, Alloc(1));
-    std::vector<bool, Alloc> expected(200, true, Alloc(1));
-    std::fill(in.begin(), in.end(), true);
-    assert(in == expected);
-  }
-  {
-    using Alloc = sized_allocator<bool, std::uint32_t, std::int32_t>;
-    std::vector<bool, Alloc> in(200, false, Alloc(1));
-    std::vector<bool, Alloc> expected(200, true, Alloc(1));
-    std::fill(in.begin(), in.end(), true);
-    assert(in == expected);
+TEST_CONSTEXPR_CXX20 bool test_vector_bool(std::size_t N) {
+  {   // Test cases validating leading/trailing bits unfilled remain unchanged
+    { // Leading bits are not filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      expected[0] = expected[1] = false;
+      std::fill(in.begin() + 2, in.end(), true);
+      assert(in == expected);
+    }
+    { // Trailing bits are not filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      expected[N - 1] = expected[N - 2] = false;
+      std::fill(in.begin(), in.end() - 2, true);
+      assert(in == expected);
+    }
+    { // Leading and trailing bits are not filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      expected[0] = expected[1] = expected[N - 1] = expected[N - 2] = false;
+      std::fill(in.begin() + 2, in.end() - 2, true);
+      assert(in == expected);
+    }
   }
-  {
-    using Alloc = sized_allocator<bool, std::uint64_t, std::int64_t>;
-    std::vector<bool, Alloc> in(200, false, Alloc(1));
-    std::vector<bool, Alloc> expected(200, true, Alloc(1));
-    std::fill(in.begin(), in.end(), true);
-    assert(in == expected);
+
+  {   // Test cases with full or partial bytes filled
+    { // Full bytes filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      std::fill(in.begin(), in.end(), true);
+      assert(in == expected);
+    }
+    { // Partial bytes with offset filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      std::fill(in.begin() + 4, in.end() - 4, true);
+      std::fill(expected.begin(), expected.begin() + 4, false);
+      std::fill(expected.end() - 4, expected.end(), false);
+      assert(in == expected);
+    }
   }
+
+  return true;
 }
 
 TEST_CONSTEXPR_CXX20 bool test() {
   types::for_each(types::forward_iterator_list<char*>(), Test<char>());
   types::for_each(types::forward_iterator_list<int*>(), Test<int>());
-  {   // test vector<bool>::iterator optimization
-    { // simple case
-      std::vector<bool> in(4, false);
-      std::vector<bool> expected(4, true);
+
+  { // Test vector<bool>::iterator optimization
+    assert(test_vector_bool(8));
+    assert(test_vector_bool(19));
+    assert(test_vector_bool(32));
+    assert(test_vector_bool(49));
+    assert(test_vector_bool(64));
+    assert(test_vector_bool(199));
+    assert(test_vector_bool(256));
+
+    // Make sure std::fill behaves properly with std::vector<bool> iterators with custom size types.
+    // See https://github.com/llvm/llvm-project/pull/122410.
+    {
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(100, false, Alloc(1));
+      std::vector<bool, Alloc> expected(100, true, Alloc(1));
       std::fill(in.begin(), in.end(), true);
       assert(in == expected);
     }
-    { // partial byte in the front is not filled
-      std::vector<bool> in(8, false);
-      std::vector<bool> expected(8, true);
-      expected[0] = false;
-      expected[1] = false;
-      std::fill(in.begin() + 2, in.end(), true);
-      assert(in == expected);
-    }
-    { // partial byte in the back is not filled
-      std::vector<bool> in(8, false);
-      std::vector<bool> expected(8, true);
-      expected[6] = false;
-      expected[7] = false;
-      std::fill(in.begin(), in.end() - 2, true);
+    {
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(200, false, Alloc(1));
+      std::vector<bool, Alloc> expected(200, true, Alloc(1));
+      std::fill(in.begin(), in.end(), true);
       assert(in == expected);
     }
-    { // partial byte in the front and back is not filled
-      std::vector<bool> in(16, false);
-      std::vector<bool> expected(16, true);
-      expected[0]  = false;
-      expected[1]  = false;
-      expected[14] = false;
-      expected[15] = false;
-      std::fill(in.begin() + 2, in.end() - 2, true);
+    {
+      using Alloc = sized_allocator<bool, std::uint32_t, std::int32_t>;
+      std::vector<bool, Alloc> in(200, false, Alloc(1));
+      std::vector<bool, Alloc> expected(200, true, Alloc(1));
+      std::fill(in.begin(), in.end(), true);
       assert(in == expected);
     }
-    { // only a few bits of a byte are set
-      std::vector<bool> in(8, false);
-      std::vector<bool> expected(8, true);
-      expected[0] = false;
-      expected[1] = false;
-      expected[6] = false;
-      expected[7] = false;
-      std::fill(in.begin() + 2, in.end() - 2, true);
+    {
+      using Alloc = sized_allocator<bool, std::uint64_t, std::int64_t>;
+      std::vector<bool, Alloc> in(200, false, Alloc(1));
+      std::vector<bool, Alloc> expected(200, true, Alloc(1));
+      std::fill(in.begin(), in.end(), true);
       assert(in == expected);
     }
   }
 
-  test_bititer_with_custom_sized_types();
-
   return true;
 }
 
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill_n.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill_n.pass.cpp
index 98c412f..4dda871 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill_n.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill_n.pass.cpp
@@ -109,37 +109,49 @@ struct Storage {
   };
 };
 
-// Make sure std::fill_n behaves properly with std::vector<bool> iterators with custom size types.
-// See https://github.com/llvm/llvm-project/pull/122410.
-TEST_CONSTEXPR_CXX20 void test_bititer_with_custom_sized_types() {
-  {
-    using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
-    std::vector<bool, Alloc> in(100, false, Alloc(1));
-    std::vector<bool, Alloc> expected(100, true, Alloc(1));
-    std::fill_n(in.begin(), in.size(), true);
-    assert(in == expected);
-  }
-  {
-    using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
-    std::vector<bool, Alloc> in(200, false, Alloc(1));
-    std::vector<bool, Alloc> expected(200, true, Alloc(1));
-    std::fill_n(in.begin(), in.size(), true);
-    assert(in == expected);
-  }
-  {
-    using Alloc = sized_allocator<bool, std::uint32_t, std::int32_t>;
-    std::vector<bool, Alloc> in(200, false, Alloc(1));
-    std::vector<bool, Alloc> expected(200, true, Alloc(1));
-    std::fill_n(in.begin(), in.size(), true);
-    assert(in == expected);
+TEST_CONSTEXPR_CXX20 bool test_vector_bool(std::size_t N) {
+  {   // Test cases validating leading/trailing bits unfilled remain unchanged
+    { // Leading bits are not filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      expected[0] = expected[1] = false;
+      std::fill_n(in.begin() + 2, N - 2, true);
+      assert(in == expected);
+    }
+    { // Trailing bits are not filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      expected[N - 1] = expected[N - 2] = false;
+      std::fill_n(in.begin(), N - 2, true);
+      assert(in == expected);
+    }
+    { // Leading and trailing bits are not filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      expected[0] = expected[1] = expected[N - 1] = expected[N - 2] = false;
+      std::fill_n(in.begin() + 2, N - 4, true);
+      assert(in == expected);
+    }
   }
-  {
-    using Alloc = sized_allocator<bool, std::uint64_t, std::int64_t>;
-    std::vector<bool, Alloc> in(200, false, Alloc(1));
-    std::vector<bool, Alloc> expected(200, true, Alloc(1));
-    std::fill_n(in.begin(), in.size(), true);
-    assert(in == expected);
+
+  {   // Test cases with full or partial bytes filled
+    { // Full bytes filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      std::fill_n(in.begin(), N, true);
+      assert(in == expected);
+    }
+    { // Partial bytes with offset filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      std::fill_n(in.begin() + 4, N - 8, true);
+      std::fill_n(expected.begin(), 4, false);
+      std::fill_n(expected.end() - 4, 4, false);
+      assert(in == expected);
+    }
   }
+
+  return true;
 }
 
 TEST_CONSTEXPR_CXX20 void test_struct_array() {
@@ -171,7 +183,47 @@ TEST_CONSTEXPR_CXX20 bool test() {
   test_int_array();
   test_struct_array();
   test_int_array_struct_source();
-  test_bititer_with_custom_sized_types();
+
+  { // Test vector<bool>::iterator optimization
+    assert(test_vector_bool(8));
+    assert(test_vector_bool(19));
+    assert(test_vector_bool(32));
+    assert(test_vector_bool(49));
+    assert(test_vector_bool(64));
+    assert(test_vector_bool(199));
+    assert(test_vector_bool(256));
+
+    // Make sure std::fill_n behaves properly with std::vector<bool> iterators with custom size types.
+    // See https://github.com/llvm/llvm-project/pull/122410.
+    {
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(100, false, Alloc(1));
+      std::vector<bool, Alloc> expected(100, true, Alloc(1));
+      std::fill_n(in.begin(), in.size(), true);
+      assert(in == expected);
+    }
+    {
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(200, false, Alloc(1));
+      std::vector<bool, Alloc> expected(200, true, Alloc(1));
+      std::fill_n(in.begin(), in.size(), true);
+      assert(in == expected);
+    }
+    {
+      using Alloc = sized_allocator<bool, std::uint32_t, std::int32_t>;
+      std::vector<bool, Alloc> in(200, false, Alloc(1));
+      std::vector<bool, Alloc> expected(200, true, Alloc(1));
+      std::fill_n(in.begin(), in.size(), true);
+      assert(in == expected);
+    }
+    {
+      using Alloc = sized_allocator<bool, std::uint64_t, std::int64_t>;
+      std::vector<bool, Alloc> in(200, false, Alloc(1));
+      std::vector<bool, Alloc> expected(200, true, Alloc(1));
+      std::fill_n(in.begin(), in.size(), true);
+      assert(in == expected);
+    }
+  }
 
   return true;
 }
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill.pass.cpp
index 30dfdd5..61a659f 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill.pass.cpp
@@ -78,42 +78,53 @@ constexpr void test_iterators() {
   }
 }
 
-// Make sure std::ranges::fill behaves properly with std::vector<bool> iterators with custom
-// size types. See https://github.com/llvm/llvm-project/pull/122410.
-//
 // The `ranges::{fill, fill_n}` algorithms require `vector<bool, Alloc>::iterator` to satisfy
 // the `std::indirectly_writable` concept when used with `vector<bool, Alloc>`, which is only
 // satisfied since C++23.
 #if TEST_STD_VER >= 23
-TEST_CONSTEXPR_CXX20 void test_bititer_with_custom_sized_types() {
-  {
-    using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
-    std::vector<bool, Alloc> in(100, false, Alloc(1));
-    std::vector<bool, Alloc> expected(100, true, Alloc(1));
-    std::ranges::fill(in, true);
-    assert(in == expected);
-  }
-  {
-    using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
-    std::vector<bool, Alloc> in(200, false, Alloc(1));
-    std::vector<bool, Alloc> expected(200, true, Alloc(1));
-    std::ranges::fill(in, true);
-    assert(in == expected);
-  }
-  {
-    using Alloc = sized_allocator<bool, std::uint32_t, std::int32_t>;
-    std::vector<bool, Alloc> in(200, false, Alloc(1));
-    std::vector<bool, Alloc> expected(200, true, Alloc(1));
-    std::ranges::fill(in, true);
-    assert(in == expected);
+constexpr bool test_vector_bool(std::size_t N) {
+  {   // Test cases validating leading/trailing bits unfilled remain unchanged
+    { // Leading bits are not filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      expected[0] = expected[1] = false;
+      std::ranges::fill(std::ranges::subrange(in.begin() + 2, in.end()), true);
+      assert(in == expected);
+    }
+    { // Trailing bits are not filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      expected[N - 1] = expected[N - 2] = false;
+      std::ranges::fill(std::ranges::subrange(in.begin(), in.end() - 2), true);
+      assert(in == expected);
+    }
+    { // Leading and trailing bits are not filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      expected[0] = expected[1] = expected[N - 1] = expected[N - 2] = false;
+      std::ranges::fill(std::ranges::subrange(in.begin() + 2, in.end() - 2), true);
+      assert(in == expected);
+    }
   }
-  {
-    using Alloc = sized_allocator<bool, std::uint64_t, std::int64_t>;
-    std::vector<bool, Alloc> in(200, false, Alloc(1));
-    std::vector<bool, Alloc> expected(200, true, Alloc(1));
-    std::ranges::fill(in, true);
-    assert(in == expected);
+
+  {   // Test cases with full or partial bytes filled
+    { // Full bytes filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      std::ranges::fill(in, true);
+      assert(in == expected);
+    }
+    { // Partial bytes with offset filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      std::ranges::fill(std::ranges::subrange(std::ranges::begin(in) + 4, std::ranges::end(in) - 4), true);
+      std::ranges::fill(std::ranges::subrange(std::ranges::begin(expected), std::ranges::begin(expected) + 4), false);
+      std::ranges::fill(std::ranges::subrange(std::ranges::end(expected) - 4, std::ranges::end(expected)), false);
+      assert(in == expected);
+    }
   }
+
+  return true;
 }
 #endif
 
@@ -174,7 +185,46 @@ constexpr bool test() {
   }
 
 #if TEST_STD_VER >= 23
-  test_bititer_with_custom_sized_types();
+  { // Test vector<bool>::iterator optimization
+    assert(test_vector_bool(8));
+    assert(test_vector_bool(19));
+    assert(test_vector_bool(32));
+    assert(test_vector_bool(49));
+    assert(test_vector_bool(64));
+    assert(test_vector_bool(199));
+    assert(test_vector_bool(256));
+
+    // Make sure std::ranges::fill behaves properly with std::vector<bool> iterators with custom
+    // size types. See https://github.com/llvm/llvm-project/pull/122410.
+    {
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(100, false, Alloc(1));
+      std::vector<bool, Alloc> expected(100, true, Alloc(1));
+      std::ranges::fill(in, true);
+      assert(in == expected);
+    }
+    {
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(200, false, Alloc(1));
+      std::vector<bool, Alloc> expected(200, true, Alloc(1));
+      std::ranges::fill(in, true);
+      assert(in == expected);
+    }
+    {
+      using Alloc = sized_allocator<bool, std::uint32_t, std::int32_t>;
+      std::vector<bool, Alloc> in(200, false, Alloc(1));
+      std::vector<bool, Alloc> expected(200, true, Alloc(1));
+      std::ranges::fill(in, true);
+      assert(in == expected);
+    }
+    {
+      using Alloc = sized_allocator<bool, std::uint64_t, std::int64_t>;
+      std::vector<bool, Alloc> in(200, false, Alloc(1));
+      std::vector<bool, Alloc> expected(200, true, Alloc(1));
+      std::ranges::fill(in, true);
+      assert(in == expected);
+    }
+  }
 #endif
 
   return true;
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill_n.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill_n.pass.cpp
index ae70e71..2d6e24a 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill_n.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill_n.pass.cpp
@@ -51,42 +51,53 @@ constexpr void test_iterators() {
   }
 }
 
-// Make sure std::ranges::fill_n behaves properly with std::vector<bool> iterators with custom
-// size types. See https://github.com/llvm/llvm-project/pull/122410.
-//
 // The `ranges::{fill, fill_n}` algorithms require `vector<bool, Alloc>::iterator` to satisfy
 // the `std::indirectly_writable` concept when used with `vector<bool, Alloc>`, which is only
 // satisfied since C++23.
 #if TEST_STD_VER >= 23
-TEST_CONSTEXPR_CXX20 void test_bititer_with_custom_sized_types() {
-  {
-    using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
-    std::vector<bool, Alloc> in(100, false, Alloc(1));
-    std::vector<bool, Alloc> expected(100, true, Alloc(1));
-    std::ranges::fill_n(std::ranges::begin(in), in.size(), true);
-    assert(in == expected);
-  }
-  {
-    using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
-    std::vector<bool, Alloc> in(200, false, Alloc(1));
-    std::vector<bool, Alloc> expected(200, true, Alloc(1));
-    std::ranges::fill_n(std::ranges::begin(in), in.size(), true);
-    assert(in == expected);
+constexpr bool test_vector_bool(std::size_t N) {
+  {   // Test cases validating leading/trailing bits unfilled remain unchanged
+    { // Leading bits are not filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      expected[0] = expected[1] = false;
+      std::ranges::fill_n(std::ranges::begin(in) + 2, N - 2, true);
+      assert(in == expected);
+    }
+    { // Trailing bits are not filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      expected[N - 1] = expected[N - 2] = false;
+      std::ranges::fill_n(std::ranges::begin(in), N - 2, true);
+      assert(in == expected);
+    }
+    { // Leading and trailing bits are not filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      expected[0] = expected[1] = expected[N - 1] = expected[N - 2] = false;
+      std::ranges::fill_n(std::ranges::begin(in) + 2, N - 4, true);
+      assert(in == expected);
+    }
   }
-  {
-    using Alloc = sized_allocator<bool, std::uint32_t, std::int32_t>;
-    std::vector<bool, Alloc> in(200, false, Alloc(1));
-    std::vector<bool, Alloc> expected(200, true, Alloc(1));
-    std::ranges::fill_n(std::ranges::begin(in), in.size(), true);
-    assert(in == expected);
-  }
-  {
-    using Alloc = sized_allocator<bool, std::uint64_t, std::int64_t>;
-    std::vector<bool, Alloc> in(200, false, Alloc(1));
-    std::vector<bool, Alloc> expected(200, true, Alloc(1));
-    std::ranges::fill_n(std::ranges::begin(in), in.size(), true);
-    assert(in == expected);
+
+  {   // Test cases with full or partial bytes filled
+    { // Full bytes filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      std::ranges::fill_n(std::ranges::begin(in), N, true);
+      assert(in == expected);
+    }
+    { // Partial bytes with offset filled
+      std::vector<bool> in(N, false);
+      std::vector<bool> expected(N, true);
+      std::ranges::fill_n(std::ranges::begin(in) + 4, N - 8, true);
+      std::ranges::fill_n(std::ranges::begin(expected), 4, false);
+      std::ranges::fill_n(std::ranges::end(expected) - 4, 4, false);
+      assert(in == expected);
+    }
   }
+
+  return true;
 }
 #endif
 
@@ -122,7 +133,46 @@ constexpr bool test() {
   }
 
 #if TEST_STD_VER >= 23
-  test_bititer_with_custom_sized_types();
+  { // Test vector<bool>::iterator optimization
+    assert(test_vector_bool(8));
+    assert(test_vector_bool(19));
+    assert(test_vector_bool(32));
+    assert(test_vector_bool(49));
+    assert(test_vector_bool(64));
+    assert(test_vector_bool(199));
+    assert(test_vector_bool(256));
+
+    // Make sure std::ranges::fill_n behaves properly with std::vector<bool> iterators with custom
+    // size types. See https://github.com/llvm/llvm-project/pull/122410.
+    {
+      using Alloc = sized_allocator<bool, std::uint8_t, std::int8_t>;
+      std::vector<bool, Alloc> in(100, false, Alloc(1));
+      std::vector<bool, Alloc> expected(100, true, Alloc(1));
+      std::ranges::fill_n(std::ranges::begin(in), in.size(), true);
+      assert(in == expected);
+    }
+    {
+      using Alloc = sized_allocator<bool, std::uint16_t, std::int16_t>;
+      std::vector<bool, Alloc> in(200, false, Alloc(1));
+      std::vector<bool, Alloc> expected(200, true, Alloc(1));
+      std::ranges::fill_n(std::ranges::begin(in), in.size(), true);
+      assert(in == expected);
+    }
+    {
+      using Alloc = sized_allocator<bool, std::uint32_t, std::int32_t>;
+      std::vector<bool, Alloc> in(200, false, Alloc(1));
+      std::vector<bool, Alloc> expected(200, true, Alloc(1));
+      std::ranges::fill_n(std::ranges::begin(in), in.size(), true);
+      assert(in == expected);
+    }
+    {
+      using Alloc = sized_allocator<bool, std::uint64_t, std::int64_t>;
+      std::vector<bool, Alloc> in(200, false, Alloc(1));
+      std::vector<bool, Alloc> expected(200, true, Alloc(1));
+      std::ranges::fill_n(std::ranges::begin(in), in.size(), true);
+      assert(in == expected);
+    }
+  }
 #endif
 
   return true;
diff --git a/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/find.pass.cpp b/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/find.pass.cpp
index 2dbad32..dd89f2c 100644
--- a/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/find.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/find.pass.cpp
@@ -14,6 +14,8 @@
 // MSVC warning C4389: '==': signed/unsigned mismatch
 // MSVC warning C4805: '==': unsafe mix of type 'char' and type 'bool' in operation
 // ADDITIONAL_COMPILE_FLAGS(cl-style-warnings): /wd4245 /wd4305 /wd4310 /wd4389 /wd4805
+// ADDITIONAL_COMPILE_FLAGS(has-fconstexpr-steps): -fconstexpr-steps=20000000
+// ADDITIONAL_COMPILE_FLAGS(has-fconstexpr-ops-limit): -fconstexpr-ops-limit=80000000
 
 // <algorithm>
 
@@ -24,6 +26,7 @@
 
 #include <algorithm>
 #include <cassert>
+#include <cstddef>
 #include <deque>
 #include <vector>
 #include <type_traits>
@@ -225,6 +228,18 @@ TEST_CONSTEXPR_CXX20 bool test() {
 
   types::for_each(types::integral_types(), TestIntegerPromotions());
 
+  { // Test vector<bool>::iterator optimization
+    std::vector<bool> vec(256 + 8);
+    for (ptrdiff_t i = 8; i <= 256; i *= 2) {
+      for (size_t offset = 0; offset < 8; offset += 2) {
+        std::fill(vec.begin(), vec.end(), false);
+        std::fill(vec.begin() + offset, vec.begin() + i + offset, true);
+        assert(std::find(vec.begin(), vec.end(), true) == vec.begin() + offset);
+        assert(std::find(vec.begin() + offset, vec.end(), false) == vec.begin() + offset + i);
+      }
+    }
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/ranges.find.pass.cpp b/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/ranges.find.pass.cpp
index 760ee23..46accc5 100644
--- a/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/ranges.find.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.nonmodifying/alg.find/ranges.find.pass.cpp
@@ -14,6 +14,8 @@
 // MSVC warning C4242: 'argument': conversion from 'const _Ty' to 'ElementT', possible loss of data
 // MSVC warning C4244: 'argument': conversion from 'const _Ty' to 'ElementT', possible loss of data
 // ADDITIONAL_COMPILE_FLAGS(cl-style-warnings): /wd4242 /wd4244
+// ADDITIONAL_COMPILE_FLAGS(has-fconstexpr-steps): -fconstexpr-steps=20000000
+// ADDITIONAL_COMPILE_FLAGS(has-fconstexpr-ops-limit): -fconstexpr-ops-limit=80000000
 
 // template<input_iterator I, sentinel_for<I> S, class T, class Proj = identity>
 //   requires indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T*>
@@ -26,6 +28,7 @@
 #include <algorithm>
 #include <array>
 #include <cassert>
+#include <cstddef>
 #include <deque>
 #include <ranges>
 #include <vector>
@@ -66,14 +69,14 @@ constexpr void test_iterators() {
   using ValueT = std::iter_value_t<It>;
   { // simple test
     {
-      ValueT a[] = {1, 2, 3, 4};
+      ValueT a[]                = {1, 2, 3, 4};
       std::same_as<It> auto ret = std::ranges::find(It(a), Sent(It(a + 4)), 4);
       assert(base(ret) == a + 3);
       assert(*ret == 4);
     }
     {
-      ValueT a[] = {1, 2, 3, 4};
-      auto range = std::ranges::subrange(It(a), Sent(It(a + 4)));
+      ValueT a[]                = {1, 2, 3, 4};
+      auto range                = std::ranges::subrange(It(a), Sent(It(a + 4)));
       std::same_as<It> auto ret = std::ranges::find(range, 4);
       assert(base(ret) == a + 3);
       assert(*ret == 4);
@@ -83,13 +86,13 @@ constexpr void test_iterators() {
   { // check that an empty range works
     {
       std::array<ValueT, 0> a = {};
-      auto ret = std::ranges::find(It(a.data()), Sent(It(a.data())), 1);
+      auto ret                = std::ranges::find(It(a.data()), Sent(It(a.data())), 1);
       assert(base(ret) == a.data());
     }
     {
       std::array<ValueT, 0> a = {};
-      auto range = std::ranges::subrange(It(a.data()), Sent(It(a.data())));
-      auto ret = std::ranges::find(range, 1);
+      auto range              = std::ranges::subrange(It(a.data()), Sent(It(a.data())));
+      auto ret                = std::ranges::find(range, 1);
       assert(base(ret) == a.data());
     }
   }
@@ -97,12 +100,12 @@ constexpr void test_iterators() {
   { // check that last is returned with no match
     {
       ValueT a[] = {1, 1, 1};
-      auto ret = std::ranges::find(a, a + 3, 0);
+      auto ret   = std::ranges::find(a, a + 3, 0);
       assert(ret == a + 3);
     }
     {
       ValueT a[] = {1, 1, 1};
-      auto ret = std::ranges::find(a, 0);
+      auto ret   = std::ranges::find(a, 0);
       assert(ret == a + 3);
     }
   }
@@ -146,7 +149,7 @@ constexpr bool test() {
         int comp;
         int other;
       };
-      S a[] = { {0, 0}, {0, 2}, {0, 1} };
+      S a[]    = {{0, 0}, {0, 2}, {0, 1}};
       auto ret = std::ranges::find(a, 0, &S::comp);
       assert(ret == a);
       assert(ret->comp == 0);
@@ -157,7 +160,7 @@ constexpr bool test() {
         int comp;
         int other;
       };
-      S a[] = { {0, 0}, {0, 2}, {0, 1} };
+      S a[]    = {{0, 0}, {0, 2}, {0, 1}};
       auto ret = std::ranges::find(a, a + 3, 0, &S::comp);
       assert(ret == a);
       assert(ret->comp == 0);
@@ -167,7 +170,7 @@ constexpr bool test() {
 
   {
     // check that an iterator is returned with a borrowing range
-    int a[] = {1, 2, 3, 4};
+    int a[]                     = {1, 2, 3, 4};
     std::same_as<int*> auto ret = std::ranges::find(std::views::all(a), 1);
     assert(ret == a);
     assert(*ret == 1);
@@ -176,23 +179,44 @@ constexpr bool test() {
   {
     // count invocations of the projection
     {
-      int a[] = {1, 2, 3, 4};
+      int a[]              = {1, 2, 3, 4};
       int projection_count = 0;
-      auto ret = std::ranges::find(a, a + 4, 2, [&](int i) { ++projection_count; return i; });
+      auto ret             = std::ranges::find(a, a + 4, 2, [&](int i) {
+        ++projection_count;
+        return i;
+      });
       assert(ret == a + 1);
       assert(*ret == 2);
       assert(projection_count == 2);
     }
     {
-      int a[] = {1, 2, 3, 4};
+      int a[]              = {1, 2, 3, 4};
       int projection_count = 0;
-      auto ret = std::ranges::find(a, 2, [&](int i) { ++projection_count; return i; });
+      auto ret             = std::ranges::find(a, 2, [&](int i) {
+        ++projection_count;
+        return i;
+      });
       assert(ret == a + 1);
       assert(*ret == 2);
       assert(projection_count == 2);
     }
   }
 
+  { // Test vector<bool>::iterator optimization
+    std::vector<bool> vec(256 + 8);
+    for (ptrdiff_t i = 8; i <= 256; i *= 2) {
+      for (size_t offset = 0; offset < 8; offset += 2) {
+        std::fill(vec.begin(), vec.end(), false);
+        std::fill(vec.begin() + offset, vec.begin() + i + offset, true);
+
+        // check both range and iterator-sentinel overloads
+        assert(std::ranges::find(vec, true) == std::ranges::begin(vec) + offset);
+        assert(std::ranges::find(std::ranges::begin(vec) + offset, std::ranges::end(vec), false) ==
+               std::ranges::begin(vec) + offset + i);
+      }
+    }
+  }
+
   return true;
 }
 
@@ -208,9 +232,7 @@ public:
           return size;
         }()) {}
 
-  bool operator==(const Comparable& other) const {
-    return comparable_data[other.index_] == comparable_data[index_];
-  }
+  bool operator==(const Comparable& other) const { return comparable_data[other.index_] == comparable_data[index_]; }
 
   friend bool operator==(const Comparable& lhs, long long rhs) { return comparable_data[lhs.index_] == rhs; }
 };