[libc++] Mark vector<bool>::at() as constexpr to conform to C++20 standard (#121848)

Closes #121844.

libcxx/include/__vector/vector_bool.h

@@ -279,8 +279,8 @@ public:
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference operator[](size_type __n) const {
     return __make_ref(__n);
   }
-  _LIBCPP_HIDE_FROM_ABI reference at(size_type __n);
-  _LIBCPP_HIDE_FROM_ABI const_reference at(size_type __n) const;
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference at(size_type __n);
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference at(size_type __n) const;
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference front() { return __make_ref(0); }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference front() const { return __make_ref(0); }
@@ -853,14 +853,15 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 void vector<bool, _Allocator>::shrink_to_fit() _NO
 }
 
 template <class _Allocator>
-typename vector<bool, _Allocator>::reference vector<bool, _Allocator>::at(size_type __n) {
+_LIBCPP_CONSTEXPR_SINCE_CXX20 typename vector<bool, _Allocator>::reference vector<bool, _Allocator>::at(size_type __n) {
   if (__n >= size())
     this->__throw_out_of_range();
   return (*this)[__n];
 }
 
 template <class _Allocator>
-typename vector<bool, _Allocator>::const_reference vector<bool, _Allocator>::at(size_type __n) const {
+_LIBCPP_CONSTEXPR_SINCE_CXX20 typename vector<bool, _Allocator>::const_reference
+vector<bool, _Allocator>::at(size_type __n) const {
   if (__n >= size())
     this->__throw_out_of_range();
   return (*this)[__n];

libcxx/test/std/containers/sequences/vector.bool/at.pass.cpp

@@ -0,0 +1,125 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <vector>
+
+// reference at(size_type n); // constexpr since C++20
+
+#include <cassert>
+#include <memory>
+#include <vector>
+
+#include "min_allocator.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+#  include <stdexcept>
+#endif
+
+template <typename Allocator>
+TEST_CONSTEXPR_CXX20 void test() {
+  using C         = std::vector<bool, Allocator>;
+  using reference = typename C::reference;
+  bool a[]        = {1, 0, 1, 0, 1};
+  C v(a, a + sizeof(a) / sizeof(a[0]));
+  ASSERT_SAME_TYPE(reference, decltype(v.at(0)));
+  assert(v.at(0) == true);
+  assert(v.at(1) == false);
+  assert(v.at(2) == true);
+  assert(v.at(3) == false);
+  assert(v.at(4) == true);
+  v.at(1) = 1;
+  assert(v.at(1) == true);
+  v.at(3) = 1;
+  assert(v.at(3) == true);
+}
+
+template <typename Allocator>
+void test_exception() {
+#ifndef TEST_HAS_NO_EXCEPTIONS
+  {
+    bool a[] = {1, 0, 1, 1};
+    using C  = std::vector<bool, Allocator>;
+    C v(a, a + sizeof(a) / sizeof(a[0]));
+
+    try {
+      TEST_IGNORE_NODISCARD v.at(4);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+
+    try {
+      TEST_IGNORE_NODISCARD v.at(5);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+
+    try {
+      TEST_IGNORE_NODISCARD v.at(6);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+
+    try {
+      using size_type = typename C::size_type;
+      TEST_IGNORE_NODISCARD v.at(static_cast<size_type>(-1));
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+  }
+
+  {
+    std::vector<bool, Allocator> v;
+    try {
+      TEST_IGNORE_NODISCARD v.at(0);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+  }
+#endif
+}
+
+TEST_CONSTEXPR_CXX20 bool tests() {
+  test<std::allocator<bool> >();
+  test<min_allocator<bool> >();
+  test<test_allocator<bool> >();
+  return true;
+}
+
+void test_exceptions() {
+  test_exception<std::allocator<bool> >();
+  test_exception<min_allocator<bool> >();
+  test_exception<test_allocator<bool> >();
+}
+
+int main(int, char**) {
+  tests();
+  test_exceptions();
+
+#if TEST_STD_VER >= 20
+  static_assert(tests());
+#endif
+
+  return 0;
+}

libcxx/test/std/containers/sequences/vector.bool/at_const.pass.cpp

@@ -0,0 +1,121 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <vector>
+
+// const_reference at(size_type n) const; // constexpr since C++20
+
+#include <cassert>
+#include <memory>
+#include <vector>
+
+#include "min_allocator.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+#  include <stdexcept>
+#endif
+
+template <typename Allocator>
+TEST_CONSTEXPR_CXX20 void test() {
+  using C               = const std::vector<bool, Allocator>;
+  using const_reference = typename C::const_reference;
+  bool a[]              = {1, 0, 1, 0, 1};
+  C v(a, a + sizeof(a) / sizeof(a[0]));
+  ASSERT_SAME_TYPE(const_reference, decltype(v.at(0)));
+  assert(v.at(0) == true);
+  assert(v.at(1) == false);
+  assert(v.at(2) == true);
+  assert(v.at(3) == false);
+  assert(v.at(4) == true);
+}
+
+template <typename Allocator>
+void test_exception() {
+#ifndef TEST_HAS_NO_EXCEPTIONS
+  {
+    bool a[] = {1, 0, 1, 1};
+    using C  = const std::vector<bool, Allocator>;
+    C v(a, a + sizeof(a) / sizeof(a[0]));
+
+    try {
+      TEST_IGNORE_NODISCARD v.at(4);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+
+    try {
+      TEST_IGNORE_NODISCARD v.at(5);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+
+    try {
+      TEST_IGNORE_NODISCARD v.at(6);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+
+    try {
+      using size_type = typename C::size_type;
+      TEST_IGNORE_NODISCARD v.at(static_cast<size_type>(-1));
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+  }
+
+  {
+    std::vector<bool, Allocator> v;
+    try {
+      TEST_IGNORE_NODISCARD v.at(0);
+      assert(false);
+    } catch (std::out_of_range const&) {
+      // pass
+    } catch (...) {
+      assert(false);
+    }
+  }
+#endif
+}
+
+TEST_CONSTEXPR_CXX20 bool tests() {
+  test<std::allocator<bool> >();
+  test<min_allocator<bool> >();
+  test<test_allocator<bool> >();
+  return true;
+}
+
+void test_exceptions() {
+  test_exception<std::allocator<bool> >();
+  test_exception<min_allocator<bool> >();
+  test_exception<test_allocator<bool> >();
+}
+
+int main(int, char**) {
+  tests();
+  test_exceptions();
+
+#if TEST_STD_VER >= 20
+  static_assert(tests());
+#endif
+
+  return 0;
+}