text: ///////////////////////////////////////////////////////////////////////////////
  //
  // Copyright (c) 2015 Microsoft Corporation. All rights reserved.
  //
  // This code is licensed under the MIT License (MIT).
  //
  // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  // THE SOFTWARE.
  //
  ///////////////////////////////////////////////////////////////////////////////
  
  #include <gtest/gtest.h>
  
  #include <gsl/gsl_util> // for at
  
  #include <array>            // for array
  #include <cstddef>          // for size_t
  #include <initializer_list> // for initializer_list
  #include <vector>           // for vector
  
  namespace
  {
      static constexpr char deathstring[] = "Expected Death";
  }
  
  TEST(at_tests, static_array)
  {
      int a[4] = {1, 2, 3, 4};
      const int(&c_a)[4] = a;
  
      for (int i = 0; i < 4; ++i) {
          EXPECT_TRUE(&gsl::at(a, i) == &a[i]);
          EXPECT_TRUE(&gsl::at(c_a, i) == &a[i]);
      }
  
      std::set_terminate([] {
          std::cerr << "Expected Death. static_array";
          std::abort();
      });
  
      EXPECT_DEATH(gsl::at(a, -1), deathstring);
      EXPECT_DEATH(gsl::at(a, 4), deathstring);
      EXPECT_DEATH(gsl::at(c_a, -1), deathstring);
      EXPECT_DEATH(gsl::at(c_a, 4), deathstring);
  }
  
  TEST(at_tests, std_array)
  {
      std::array<int, 4> a = {1, 2, 3, 4};
      const std::array<int, 4>& c_a = a;
  
      for (int i = 0; i < 4; ++i) {
          EXPECT_TRUE(&gsl::at(a, i) == &a[static_cast<std::size_t>(i)]);
          EXPECT_TRUE(&gsl::at(c_a, i) == &a[static_cast<std::size_t>(i)]);
      }
  
      std::set_terminate([] {
          std::cerr << "Expected Death. std_array";
          std::abort();
      });
  
      EXPECT_DEATH(gsl::at(a, -1), deathstring);
      EXPECT_DEATH(gsl::at(a, 4), deathstring);
      EXPECT_DEATH(gsl::at(c_a, -1), deathstring);
      EXPECT_DEATH(gsl::at(c_a, 4), deathstring);
  }
  
  TEST(at_tests, std_vector)
  {
      std::vector<int> a = {1, 2, 3, 4};
      const std::vector<int>& c_a = a;
  
      for (int i = 0; i < 4; ++i) {
          EXPECT_TRUE(&gsl::at(a, i) == &a[static_cast<std::size_t>(i)]);
          EXPECT_TRUE(&gsl::at(c_a, i) == &a[static_cast<std::size_t>(i)]);
      }
  
      std::set_terminate([] {
          std::cerr << "Expected Death. std_vector";
          std::abort();
      });
  
      EXPECT_DEATH(gsl::at(a, -1), deathstring);
      EXPECT_DEATH(gsl::at(a, 4), deathstring);
      EXPECT_DEATH(gsl::at(c_a, -1), deathstring);
      EXPECT_DEATH(gsl::at(c_a, 4), deathstring);
  }
  
  TEST(at_tests, InitializerList)
  {
      const std::initializer_list<int> a = {1, 2, 3, 4};
  
      for (int i = 0; i < 4; ++i) {
          EXPECT_TRUE(gsl::at(a, i) == i + 1);
          EXPECT_TRUE(gsl::at({1, 2, 3, 4}, i) == i + 1);
      }
  
      std::set_terminate([] {
          std::cerr << "Expected Death. InitializerList";
          std::abort();
      });
  
      EXPECT_DEATH(gsl::at(a, -1), deathstring);
      EXPECT_DEATH(gsl::at(a, 4), deathstring);
      EXPECT_DEATH(gsl::at({1, 2, 3, 4}, -1), deathstring);
      EXPECT_DEATH(gsl::at({1, 2, 3, 4}, 4), deathstring);
  }
  
  #if !defined(_MSC_VER) || defined(__clang__) || _MSC_VER >= 1910
  static constexpr bool test_constexpr()
  {
      int a1[4] = {1, 2, 3, 4};
      const int(&c_a1)[4] = a1;
      std::array<int, 4> a2 = {1, 2, 3, 4};
      const std::array<int, 4>& c_a2 = a2;
  
      for (int i = 0; i < 4; ++i) {
          if (&gsl::at(a1, i) != &a1[i]) return false;
          if (&gsl::at(c_a1, i) != &a1[i]) return false;
          // requires C++17:
          // if (&gsl::at(a2, i) != &a2[static_cast<std::size_t>(i)]) return false;
          if (&gsl::at(c_a2, i) != &c_a2[static_cast<std::size_t>(i)]) return false;
          if (gsl::at({1, 2, 3, 4}, i) != i + 1) return false;
      }
  
      return true;
  }
  
  static_assert(test_constexpr(), "FAIL");
  #endif
  