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diff --git a/include/gsl/gsl_util b/include/gsl/gsl_util
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+///////////////////////////////////////////////////////////////////////////////
+//
+// 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.
+//
+///////////////////////////////////////////////////////////////////////////////
+
+#pragma once
+
+#ifndef GSL_UTIL_H
+#define GSL_UTIL_H
+
+#include <gsl/gsl_assert> // Ensures/Expects
+
+#include <array>
+#include <exception>
+#include <type_traits>
+#include <utility>
+
+#if defined(_MSC_VER)
+
+#pragma warning(push)
+#pragma warning(disable : 4127) // conditional expression is constant
+
+#if _MSC_VER < 1910
+#pragma push_macro("constexpr")
+#define constexpr /*constexpr*/
+#endif                          // _MSC_VER < 1910
+#endif                          // _MSC_VER
+
+// AFAIK there's no way to detect in GCC or MSVC. GCC has an __EXCEPTIONS macro,
+// but that is only defined if `-fexceptions` is *explicitly* passed on the
+// command line; defaulting to exceptions enabled will not set this macro.
+//
+// Non-Clang users will need to defined GSL_NO_EXCEPTIONS explicitly from the
+// command line if they are building with exceptions disabled.
+#if defined(__clang__) && !__has_feature(cxx_exceptions)
+// If building with -fno-exceptions, we'll fall back to gsl::Ensures in places
+// where we would have normally thrown an exception.
+#define GSL_NO_EXCEPTIONS
+#endif // defined(__has_feature) && !__has_feature(cxx_exceptions)
+
+namespace gsl
+{
+//
+// GSL.util: utilities
+//
+
+// final_act allows you to ensure something gets run at the end of a scope
+template <class F>
+class final_act
+{
+public:
+    explicit final_act(F f) noexcept : f_(std::move(f)), invoke_(true) {}
+
+    final_act(final_act&& other) noexcept : f_(std::move(other.f_)), invoke_(other.invoke_)
+    {
+        other.invoke_ = false;
+    }
+
+    final_act(const final_act&) = delete;
+    final_act& operator=(const final_act&) = delete;
+
+    ~final_act() noexcept
+    {
+        if (invoke_) f_();
+    }
+
+private:
+    F f_;
+    bool invoke_;
+};
+
+// finally() - convenience function to generate a final_act
+template <class F>
+inline final_act<F> finally(const F& f) noexcept
+{
+    return final_act<F>(f);
+}
+
+template <class F>
+inline final_act<F> finally(F&& f) noexcept
+{
+    return final_act<F>(std::forward<F>(f));
+}
+
+// narrow_cast(): a searchable way to do narrowing casts of values
+template <class T, class U>
+inline constexpr T narrow_cast(U&& u) noexcept
+{
+    return static_cast<T>(std::forward<U>(u));
+}
+
+struct narrowing_error : public std::exception
+{
+};
+
+namespace details
+{
+    template <class T, class U>
+    struct is_same_signedness
+        : public std::integral_constant<bool, std::is_signed<T>::value == std::is_signed<U>::value>
+    {
+    };
+}
+
+// narrow() : a checked version of narrow_cast() that throws if the cast changed the value
+template <class T, class U>
+inline T narrow(U u)
+{
+    T t = narrow_cast<T>(u);
+#ifndef GSL_NO_EXCEPTIONS
+    if (static_cast<U>(t) != u) throw narrowing_error();
+    if (!details::is_same_signedness<T, U>::value && ((t < T{}) != (u < U{})))
+        throw narrowing_error();
+#else
+    Ensures(static_cast<U>(t) == u);
+    Ensures(!(!details::is_same_signedness<T, U>::value && ((t < T{}) != (u < U{}))));
+#endif
+    return t;
+}
+
+//
+// at() - Bounds-checked way of accessing builtin arrays, std::array, std::vector
+//
+template <class T, std::size_t N>
+inline constexpr T& at(T (&arr)[N], const std::ptrdiff_t index)
+{
+    Expects(index >= 0 && index < narrow_cast<std::ptrdiff_t>(N));
+    return arr[static_cast<std::size_t>(index)];
+}
+
+template <class Cont>
+inline constexpr auto at(Cont& cont, const std::ptrdiff_t index) -> decltype(cont[cont.size()])
+{
+    Expects(index >= 0 && index < narrow_cast<std::ptrdiff_t>(cont.size()));
+    using size_type = decltype(cont.size());
+    return cont[static_cast<size_type>(index)];
+}
+
+template <class T>
+inline constexpr T at(const std::initializer_list<T> cont, const std::ptrdiff_t index)
+{
+    Expects(index >= 0 && index < narrow_cast<std::ptrdiff_t>(cont.size()));
+    return *(cont.begin() + index);
+}
+
+} // namespace gsl
+
+#if defined(_MSC_VER)
+#if _MSC_VER < 1910
+#undef constexpr
+#pragma pop_macro("constexpr")
+
+#endif // _MSC_VER < 1910
+
+#pragma warning(pop)
+
+#endif // _MSC_VER
+
+#endif // GSL_UTIL_H