diff --git a/README.md b/README.md
index 784aebc7..fbd17a0d 100644
--- a/README.md
+++ b/README.md
@@ -41,7 +41,7 @@ span_p                                                                   | &#x26
 [cu32zstring](docs/headers.md#user-content-H-zstring)                    | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `const char32_t`
 [**2. Owners**][cg-owners]                                               |            |
 stack_array                                                              | ☐   | A stack-allocated array
-dyn_array                                                                | ☐   | A heap-allocated array
+dyn_array                                                                | ☑   | A heap-allocated array
 [**3. Assertions**][cg-assertions]                                       |            |
 [Expects](docs/headers.md#user-content-H-assert-expects)                 | ☑   | A precondition assertion; on failure it terminates
 [Ensures](docs/headers.md#user-content-H-assert-ensures)                 | ☑   | A postcondition assertion; on failure it terminates
diff --git a/docs/headers.md b/docs/headers.md
index e8372f7c..9fd79d26 100644
--- a/docs/headers.md
+++ b/docs/headers.md
@@ -9,6 +9,7 @@ See [GSL: Guidelines support library](https://isocpp.github.io/CppCoreGuidelines
 - [`<algorithms>`](#user-content-H-algorithms)
 - [`<assert>`](#user-content-H-assert)
 - [`<byte>`](#user-content-H-byte)
+- [`<dyn_array>`](#user-content-H-dyn_array)
 - [`<gsl>`](#user-content-H-gsl)
 - [`<narrow>`](#user-content-H-narrow)
 - [`<pointers>`](#user-content-H-pointers)
@@ -155,6 +156,10 @@ constexpr byte to_byte() noexcept;
 
 Convert the given value `I` to a `byte`. The template requires `I` to be in the valid range 0..255 for a `gsl::byte`.
 
+## <a name="H-dyn_array" />`<dyn_array>`
+
+# TODO (@carsonradtke)
+
 ## <a name="H-gsl" />`<gsl>`
 
 This header is a convenience header that includes all other [GSL headers](#user-content-H).
diff --git a/include/gsl/dyn_array b/include/gsl/dyn_array
new file mode 100644
index 00000000..d9249f33
--- /dev/null
+++ b/include/gsl/dyn_array
@@ -0,0 +1,352 @@
+// -*- C++ -*-
+///////////////////////////////////////////////////////////////////////////////
+//
+// Copyright (c) 2026 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.
+//
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef GSL_DYN_ARRAY_H
+#define GSL_DYN_ARRAY_H
+
+#include "./assert"
+#include "./narrow"
+#include "./util"
+
+#include <algorithm>
+#include <iterator>
+#include <memory>
+
+#ifdef GSL_HAS_RANGES
+#include <ranges>
+#endif /* GSL_HAS_RANGES */
+
+namespace gsl
+{
+namespace details
+{
+    template <typename T>
+    struct dyn_array_traits
+    {
+        using value_type = T;
+        using pointer = T*;
+        using reference = T&;
+        using const_reference = const T&;
+        using difference_type = std::ptrdiff_t;
+        using size_type = decltype(sizeof(0));
+    };
+
+    template <typename T, typename Allocator = std::allocator<T>>
+    class dyn_array_base : public Allocator
+    {
+        using pointer = typename dyn_array_traits<T>::pointer;
+        using size_type = typename dyn_array_traits<T>::size_type;
+
+        const class dyn_array_impl
+        {
+        public:
+            constexpr dyn_array_impl(pointer data, size_type count) : _data{data}, _count{count}
+            {
+                Ensures((_count == 0 && _data == nullptr) || (_count > 0 && _data != nullptr));
+            }
+
+            constexpr auto data() const { return _data; }
+
+            constexpr auto count() const { return _count; }
+
+        private:
+            pointer _data;
+            size_type _count;
+        } _impl;
+
+    public:
+        constexpr dyn_array_base(const Allocator& alloc) : Allocator{alloc}, _impl{nullptr, 0} {}
+        constexpr dyn_array_base(size_type count, const Allocator& alloc)
+            : Allocator{alloc}, _impl{count == 0 ? nullptr : Allocator::allocate(count), count}
+        {}
+
+        GSL_CONSTEXPR_SINCE_CPP20 ~dyn_array_base()
+        {
+            if (impl().data()) Allocator::deallocate(impl().data(), impl().count());
+        }
+
+        constexpr auto impl() const -> const dyn_array_impl& { return _impl; }
+    };
+
+    template <typename T>
+    class dyn_array_iterator
+    {
+        using size_type = typename dyn_array_traits<T>::size_type;
+
+    public:
+        using difference_type = typename dyn_array_traits<T>::difference_type;
+        using value_type = typename dyn_array_traits<T>::value_type;
+        using pointer = typename dyn_array_traits<T>::pointer;
+        using reference = typename dyn_array_traits<T>::reference;
+        using const_reference = typename dyn_array_traits<T>::const_reference;
+        using iterator_category = std::random_access_iterator_tag;
+
+#ifdef GSL_HAS_RANGES
+        constexpr dyn_array_iterator() : dyn_array_iterator(nullptr, 0, 0) {}
+#endif /* GSL_HAS_RANGES */
+
+        constexpr dyn_array_iterator(pointer ptr, size_type pos, size_type end_pos)
+            : _ptr{ptr}, _pos{pos}, _end_pos{end_pos}
+        {
+            Ensures((_ptr != nullptr && _end_pos > 0) || (_ptr == nullptr && _end_pos == 0));
+            Ensures(_pos <= _end_pos);
+        }
+
+#if defined(_MSC_VER) && defined(GSL_HAS_RANGES)
+        // TODO (@carsonradtke): Investigate why this is necessary for MSVC.
+        // Is it a a bug in GSL? STL? MSVC?
+        constexpr operator pointer() const { return _ptr + gsl::narrow<size_type>(_pos); }
+#endif /* defined(_MSC_VER) && defined(GSL_HAS_RANGES) */
+
+        constexpr auto operator==(const dyn_array_iterator& other) const
+        {
+            Expects(_ptr == other._ptr);
+            Expects(_end_pos == other._end_pos);
+            return _pos == other._pos;
+        }
+
+        constexpr auto operator!=(const dyn_array_iterator& other) const
+        {
+            return !(*this == other);
+        }
+
+        constexpr auto operator*() const -> reference
+        {
+            Expects(_ptr != nullptr);
+            Expects(_pos < _end_pos);
+            return _ptr[_pos];
+        }
+
+        constexpr auto operator++() -> dyn_array_iterator&
+        {
+            Expects(_pos < _end_pos);
+            _pos++;
+            return *this;
+        }
+
+        constexpr auto operator++(int)
+        {
+            auto rv = *this;
+            ++(*this);
+            return rv;
+        }
+
+        constexpr auto operator--() -> dyn_array_iterator&
+        {
+            Expects(_pos > 0);
+            _pos--;
+            return *this;
+        }
+
+        constexpr auto operator--(int)
+        {
+            auto rv = *this;
+            --(*this);
+            return rv;
+        }
+
+        constexpr auto operator+=(difference_type diff) -> dyn_array_iterator&
+        {
+            auto pos = gsl::narrow<difference_type>(_pos);
+            Expects(pos + diff <= gsl::narrow<difference_type>(_end_pos));
+            _pos = gsl::narrow<size_type>(pos + diff);
+            return *this;
+        }
+
+        constexpr auto operator-=(difference_type diff) -> dyn_array_iterator&
+        {
+            auto pos = gsl::narrow<difference_type>(_pos);
+            Expects(pos >= diff);
+            _pos = gsl::narrow<size_type>(pos - diff);
+            return *this;
+        }
+
+        constexpr auto operator+(difference_type diff) const
+        {
+            return dyn_array_iterator{_ptr, _pos + gsl::narrow<size_type>(diff), _end_pos};
+        }
+
+        constexpr auto operator-(difference_type diff) const
+        {
+            return dyn_array_iterator{_ptr, _pos + gsl::narrow<size_type>(diff), _end_pos};
+        }
+
+        constexpr auto operator-(const dyn_array_iterator& other) const
+        {
+            Expects(_ptr == other._ptr);
+            Expects(_end_pos == other._end_pos);
+            return gsl::narrow<difference_type>(_pos) - gsl::narrow<difference_type>(other._pos);
+        }
+
+        constexpr auto operator[](size_type pos) -> reference
+        {
+            Expects(_pos + pos < _end_pos);
+            return _ptr[_pos + pos];
+        }
+
+        constexpr auto operator[](size_type pos) const -> const_reference
+        {
+            return const_cast<dyn_array_iterator&>(*this).operator[](pos);
+        }
+
+    private:
+        pointer _ptr;
+        size_type _pos;
+        size_type _end_pos;
+    };
+} // namespace details
+
+template <typename T, typename Allocator = std::allocator<T>>
+class dyn_array : public details::dyn_array_base<T, Allocator>
+{
+    using base = details::dyn_array_base<T, Allocator>;
+    using pointer = typename details::dyn_array_traits<T>::pointer;
+
+public:
+    using value_type = typename details::dyn_array_traits<T>::value_type;
+    using reference = typename details::dyn_array_traits<T>::reference;
+    using const_reference = typename details::dyn_array_traits<T>::const_reference;
+    using iterator = details::dyn_array_iterator<T>;
+    using const_iterator = details::dyn_array_iterator<const T>;
+    using reverse_iterator = std::reverse_iterator<iterator>;
+    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+    using difference_type = typename details::dyn_array_traits<T>::difference_type;
+    using size_type = typename details::dyn_array_traits<T>::size_type;
+
+    using allocator_type = Allocator;
+
+    explicit constexpr dyn_array(const Allocator& alloc = {}) : base{alloc} {}
+
+    explicit constexpr dyn_array(size_type count, const T& value, const Allocator& alloc = {})
+        : base{count, alloc}
+    {
+        std::fill(begin(), end(), value);
+    }
+
+    GSL_TYPE_IS_ITERATOR(InputIt)
+    constexpr dyn_array(InputIt first, InputIt last, const Allocator& alloc = {})
+        : base{gsl::narrow<size_type>(std::distance(first, last)), alloc}
+    {
+        std::copy(first, last, begin());
+    }
+
+#ifdef GSL_HAS_CONTAINER_RANGES
+    template <typename InputRg>
+        requires(std::ranges::input_range<InputRg>)
+    constexpr dyn_array(std::from_range_t, InputRg&& rg, const Allocator& alloc = {})
+        : base{gsl::narrow<size_type>(std::size(rg)), alloc}
+    {
+        std::ranges::copy(rg, std::ranges::begin(*this));
+    }
+#endif /* GSL_HAS_RANGES */
+
+    constexpr explicit dyn_array(size_type count, const Allocator& alloc = {})
+        : dyn_array{count, T{}, alloc}
+    {}
+
+    constexpr dyn_array(const dyn_array& other, const Allocator& alloc = {})
+        : dyn_array(other.begin(), other.end(), alloc)
+    {}
+
+    constexpr dyn_array(std::initializer_list<T> init, const Allocator& alloc = {})
+        : dyn_array(init.begin(), init.end(), alloc)
+    {}
+
+    constexpr auto operator=(const dyn_array& other) -> dyn_array& { return dyn_array{other}; }
+
+    constexpr dyn_array(dyn_array&&) = delete;
+    dyn_array& operator=(dyn_array&&) = delete;
+
+    constexpr auto operator==(const dyn_array& other) const
+    {
+        return size() == other.size() && std::equal(begin(), end(), other.begin(), other.end());
+    }
+
+    constexpr auto operator!=(const dyn_array& other) const { return !(*this == other); }
+
+    constexpr auto size() const { return impl().count(); }
+
+    constexpr auto empty() const { return size() == 0; }
+
+    constexpr auto max_size() const { return static_cast<size_type>(-1); }
+
+    constexpr auto get_allocator() -> Allocator& { return *this; }
+
+    constexpr auto operator[](size_type pos) -> reference
+    {
+        Expects(pos < size());
+        return data()[pos];
+    }
+
+    constexpr auto operator[](size_type pos) const -> const_reference
+    {
+        return const_cast<dyn_array&>(*this)[pos];
+    }
+
+    constexpr auto data() { return impl().data(); }
+    constexpr auto data() const -> const T* { return const_cast<dyn_array&>(*this).data(); }
+
+    constexpr auto begin() { return iterator{data(), 0, size()}; }
+    constexpr auto begin() const { return const_iterator{data(), 0, size()}; }
+
+    constexpr auto rbegin() { return reverse_iterator{end()}; }
+    constexpr auto rbegin() const { return const_reverse_iterator{end()}; }
+
+#ifdef _MSC_VER
+    constexpr auto _Unchecked_begin() { return data(); }
+    constexpr auto _Unchecked_begin() const -> const pointer
+    {
+        return const_cast<dyn_array&>(*this)._Unchecked_begin();
+    }
+#endif /* _MSC_VER */
+
+    constexpr auto end() { return iterator{data(), size(), size()}; }
+    constexpr auto end() const { return const_iterator{data(), size(), size()}; }
+
+    constexpr auto rend() { return reverse_iterator{begin()}; }
+    constexpr auto rend() const { return const_reverse_iterator{begin()}; }
+
+#ifdef _MSC_VER
+    constexpr auto _Unchecked_end() { return data() + size(); }
+    constexpr auto _Unchecked_end() const -> const pointer
+    {
+        return const_cast<dyn_array&>(*this)._Unchecked_end();
+    }
+#endif /* MSC_VER */
+
+private:
+    constexpr auto impl() const { return base::impl(); }
+};
+
+#ifdef GSL_HAS_DEDUCTION_GUIDES
+
+template <class InputIt,
+          class Alloc = std::allocator<typename std::iterator_traits<InputIt>::value_type>>
+dyn_array(InputIt, InputIt,
+          Alloc = {}) -> dyn_array<typename std::iterator_traits<InputIt>::value_type, Alloc>;
+
+#ifdef GSL_HAS_CONTAINER_RANGES
+template <std::ranges::input_range InputRg,
+          class Alloc = std::allocator<std::ranges::range_value_t<InputRg>>>
+dyn_array(std::from_range_t, InputRg&&,
+          Alloc = {}) -> dyn_array<std::ranges::range_value_t<InputRg>, Alloc>;
+#endif /* GSL_HAS_RANGES */
+
+#endif /* GSL_HAS_DEDUCTION_GUIDES */
+} // namespace gsl
+
+#endif /* defined(GSL_DYN_ARRAY_H) */
diff --git a/include/gsl/pointers b/include/gsl/pointers
index 282229fe..ca8a2077 100644
--- a/include/gsl/pointers
+++ b/include/gsl/pointers
@@ -1,3 +1,4 @@
+// -*- C++ -*-
 ///////////////////////////////////////////////////////////////////////////////
 //
 // Copyright (c) 2015 Microsoft Corporation. All rights reserved.
@@ -338,7 +339,7 @@ auto make_strict_not_null(T&& t) noexcept
     return strict_not_null<std::remove_cv_t<std::remove_reference_t<T>>>{std::forward<T>(t)};
 }
 
-#if (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L))
+#ifdef GSL_HAS_DEDUCTION_GUIDES
 
 // deduction guides to prevent the ctad-maybe-unsupported warning
 template <class T>
@@ -346,7 +347,7 @@ not_null(T) -> not_null<T>;
 template <class T>
 strict_not_null(T) -> strict_not_null<T>;
 
-#endif // ( defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L) )
+#endif // GSL_HAS_DEDUCTION_GUIDES
 
 } // namespace gsl
 
diff --git a/include/gsl/span b/include/gsl/span
index cef8a774..ebad0c61 100644
--- a/include/gsl/span
+++ b/include/gsl/span
@@ -1,3 +1,4 @@
+// -*- C++ -*-
 ///////////////////////////////////////////////////////////////////////////////
 //
 // Copyright (c) 2015 Microsoft Corporation. All rights reserved.
@@ -767,7 +768,7 @@ private:
     }
 };
 
-#if (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L))
+#ifdef GSL_HAS_DEDUCTION_GUIDES
 
 // Deduction Guides
 template <class Type, std::size_t Extent>
@@ -787,7 +788,7 @@ template <class Container,
           class Element = std::remove_pointer_t<decltype(std::declval<const Container&>().data())>>
 span(const Container&) -> span<Element>;
 
-#endif // ( defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L) )
+#endif // GSL_HAS_DEDUCTION_GUIDES
 
 #if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
 #if defined(__clang__) && defined(_MSC_VER) && defined(__cplusplus) && (__cplusplus < 201703L)
diff --git a/include/gsl/util b/include/gsl/util
index 7a3caed4..0f46694a 100644
--- a/include/gsl/util
+++ b/include/gsl/util
@@ -1,3 +1,4 @@
+// -*- C++ -*-
 ///////////////////////////////////////////////////////////////////////////////
 //
 // Copyright (c) 2015 Microsoft Corporation. All rights reserved.
@@ -19,8 +20,8 @@
 
 #include "./assert" // for Expects
 
-#include <array>
 #include <cstddef>          // for ptrdiff_t, size_t
+#include <iterator>         // for is_iterator, iterator_traits
 #include <limits>           // for numeric_limits
 #include <initializer_list> // for initializer_list
 #include <type_traits>      // for is_signed, integral_constant
@@ -86,6 +87,38 @@
 #define GSL_DEPRECATED(msg)
 #endif // !defined(GSL_DEPRECATED)
 
+#if (defined(__cpp_deduction_guides) && __cpp_deduction_guides >= 201703L)
+#define GSL_HAS_DEDUCTION_GUIDES
+#endif // (defined(__cpp_deduction_guides) && __cpp_deduction_guides >= 201703L)
+
+#if (defined(__cpp_lib_ranges) && __cpp_lib_ranges >= 201911L)
+#define GSL_HAS_RANGES
+#endif // (defined(__cpp_lib_ranges) && __cpp_lib_ranges >= 201911L)
+
+#if (defined(__cpp_lib_containers_ranges) && __cpp_lib_containers_ranges >= 202202L)
+#define GSL_HAS_CONTAINER_RANGES
+#endif // (defined(__cpp_lib_containers_ranges) && __cpp_lib_containers_ranges >= 202202L)
+
+#if (defined(__cpp_lib_concepts) && __cpp_lib_concepts >= 202002L)
+#define GSL_HAS_CONCEPTS
+#endif // (defined(__cpp_lib_concepts) && __cpp_lib_concepts >= 202002L)
+
+#if (defined(__cpp_lib_constexpr_dynamic_alloc) && __cpp_lib_constexpr_dynamic_alloc >= 201907L)
+#define GSL_HAS_CONSTEXPR_ALLOCATOR
+#endif // (defined(__cpp_lib_constexpr_dynamic_alloc) && __cpp_lib_constexpr_dynamic_alloc >= 201907L)
+
+#if __cplusplus >= 202002L
+#define GSL_CONSTEXPR_SINCE_CPP20 constexpr
+#define GSL_TYPE_IS_ITERATOR(Ty)                                                                   \
+    template <typename Ty>                                                                         \
+        requires(std::input_iterator<Ty>)
+#else // ^^^ since C++20 /// before C++20 vvv
+#define GSL_UNTIL_CPP20
+#define GSL_CONSTEXPR_SINCE_CPP20
+#define GSL_TYPE_IS_ITERATOR(Ty)                                                                   \
+    template <typename Ty, std::enable_if_t<details::is_iterator<Ty>::value, bool> = true>
+#endif // __cplusplus >= 202002L
+
 namespace gsl
 {
 //
@@ -95,6 +128,22 @@ namespace gsl
 // index type for all container indexes/subscripts/sizes
 using index = std::ptrdiff_t;
 
+#ifdef GSL_UNTIL_CPP20
+namespace details
+{
+    template <typename ...>
+    using void_t = void;
+
+    template <typename T, typename = void>
+    struct is_iterator : std::false_type
+    {};
+
+   template <typename T>
+    struct is_iterator<T, void_t<typename std::iterator_traits<T>::value_type>>
+        : std::true_type {};
+} // namespace details
+#endif /* GSL_UNTIL_CPP20 */
+
 // final_action allows you to ensure something gets run at the end of a scope
 template <class F>
 class final_action
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index d583c24b..3ec624c1 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -151,6 +151,7 @@ else()
           -Wno-unknown-attributes
           -Wno-used-but-marked-unused # GTest EXPECT_DEATH
           -Wno-weak-vtables
+          -Wno-poison-system-directories
           $<$<EQUAL:${GSL_CXX_STANDARD},14>: # no support for [[maybe_unused]]
             -Wno-unused-member-function
             -Wno-unused-variable
@@ -280,6 +281,7 @@ else()
           -Wno-missing-prototypes
           -Wno-unknown-attributes
           -Wno-weak-vtables
+          -Wno-poison-system-directories
         >
         $<$<CXX_COMPILER_ID:GNU>:
           -Wdouble-promotion # float implicit to double
diff --git a/tests/dyn_array_tests.cpp b/tests/dyn_array_tests.cpp
new file mode 100644
index 00000000..8129c19b
--- /dev/null
+++ b/tests/dyn_array_tests.cpp
@@ -0,0 +1,290 @@
+#include <gtest/gtest.h>
+
+#include <gsl/dyn_array>
+#include <gsl/util>
+
+// Despite using <algorithm> and <ranges> utilities in this test, they
+// are not being included directly by this file as a test to ensure
+// transitive inclusion via <gsl/dyn_array>.
+
+static_assert(sizeof(gsl::dyn_array<int>) == 2 * sizeof(void*),
+              "gsl::dyn_array (with the default allocator) should be 16 bytes");
+
+#ifdef GSL_HAS_CONCEPTS
+static_assert(std::input_iterator<gsl::dyn_array<int>::iterator>,
+              "gsl_dyn_array should expose a valid input_iterator");
+#endif /* GSL_HAS_CONCEPTS */
+
+#ifdef GSL_HAS_RANGES
+static_assert(std::ranges::input_range<gsl::dyn_array<int>>,
+              "gsl::dyn_array should be a valid input range");
+#endif /* GSL_HAS_RANGES */
+
+TEST(dyn_array_tests, default_ctor)
+{
+    gsl::dyn_array<char> diamondbacks;
+    EXPECT_TRUE(diamondbacks.empty());
+    EXPECT_EQ(diamondbacks.size(), 0);
+    EXPECT_EQ(diamondbacks.data(), nullptr);
+}
+
+TEST(dyn_array_tests, count_ctor)
+{
+    gsl::dyn_array<char> athletics(10);
+    EXPECT_FALSE(athletics.empty());
+    EXPECT_EQ(athletics.size(), 10);
+    EXPECT_NE(athletics.data(), nullptr);
+
+    gsl::dyn_array<char> braves(0);
+    EXPECT_TRUE(braves.empty());
+    EXPECT_EQ(braves.size(), 0);
+    EXPECT_EQ(braves.data(), nullptr);
+    EXPECT_TRUE(std::all_of(braves.begin(), braves.end(), [](char c) { return c == char{}; }));
+}
+
+TEST(dyn_array_tests, count_value_ctor)
+{
+    gsl::dyn_array<char> orioles(10, 'c');
+    EXPECT_FALSE(orioles.empty());
+    EXPECT_EQ(orioles.size(), 10);
+    EXPECT_NE(orioles.data(), nullptr);
+    EXPECT_TRUE(std::all_of(orioles.begin(), orioles.end(), [](char c) { return c == 'c'; }));
+
+    gsl::dyn_array<int> redsox(10, 42);
+    EXPECT_FALSE(redsox.empty());
+    EXPECT_EQ(redsox.size(), 10);
+    EXPECT_NE(redsox.data(), nullptr);
+    EXPECT_TRUE(std::all_of(redsox.begin(), redsox.end(), [](int i) { return i == 42; }));
+}
+
+TEST(dyn_array_tests, inputit_ctor)
+{
+    std::vector<char> cubs(10, 'c');
+    gsl::dyn_array<char> whitesox(cubs.begin(), cubs.end());
+    EXPECT_FALSE(whitesox.empty());
+    EXPECT_EQ(whitesox.size(), cubs.size());
+    EXPECT_NE(whitesox.data(), nullptr);
+    EXPECT_TRUE(std::all_of(whitesox.begin(), whitesox.end(), [](char c) { return c == 'c'; }));
+}
+
+TEST(dyn_array_tests, copy_ctor)
+{
+    gsl::dyn_array<char> reds(10, 'c');
+    gsl::dyn_array<char> guardians(reds);
+    EXPECT_FALSE(guardians.empty());
+    EXPECT_EQ(guardians.size(), reds.size());
+    EXPECT_NE(guardians.data(), nullptr);
+    EXPECT_TRUE(std::all_of(guardians.begin(), guardians.end(), [](char c) { return c == 'c'; }));
+}
+
+TEST(dyn_array_tests, access_operator)
+{
+    gsl::dyn_array<char> rockies(10, 'c');
+    using ST = typename decltype(rockies)::size_type;
+    for (int i = 0; i < gsl::narrow<int>(rockies.size()); i++)
+        EXPECT_EQ(rockies[gsl::narrow<ST>(i)], 'c');
+    for (int i = 0; i < gsl::narrow<int>(rockies.size()); i++) rockies[gsl::narrow<ST>(i)] = 'r';
+    for (int i = 0; i < gsl::narrow<int>(rockies.size()); i++)
+        EXPECT_EQ(rockies[gsl::narrow<ST>(i)], 'r');
+    gsl::dyn_array<int> tigers(10);
+    for (int i = 0; i < gsl::narrow<int>(tigers.size()); i++) tigers[gsl::narrow<ST>(i)] = i;
+    for (int i = 0; i < gsl::narrow<int>(tigers.size()); i++)
+        EXPECT_EQ(tigers[gsl::narrow<ST>(i)], i);
+}
+
+TEST(dyn_array_tests, iterators)
+{
+    gsl::dyn_array<char> astros(10, 'c');
+    for (auto it = astros.begin(); it != astros.end(); it++) EXPECT_EQ(*it, 'c');
+    for (auto it = astros.begin(); it != astros.end(); it++) *it = 'r';
+    for (auto it = astros.begin(); it != astros.end(); it++) EXPECT_EQ(*it, 'r');
+    EXPECT_TRUE(std::all_of(astros.begin(), astros.end(), [](char c) { return c == 'r'; }));
+
+    gsl::dyn_array<char> royals(10, 'c');
+    for (auto it = royals.begin(); it != royals.end(); ++it) EXPECT_EQ(*it, 'c');
+    for (auto it = royals.begin(); it != royals.end(); ++it) *it = 'r';
+    for (auto it = royals.begin(); it != royals.end(); ++it) EXPECT_EQ(*it, 'r');
+    EXPECT_TRUE(std::all_of(royals.begin(), royals.end(), [](char c) { return c == 'r'; }));
+}
+
+TEST(dyn_array_tests, range_for)
+{
+    gsl::dyn_array<char> angels(10, 'c');
+    for (auto x : angels) EXPECT_EQ(x, 'c');
+    for (auto& x : angels) x = 'r';
+    for (auto x : angels) EXPECT_EQ(x, 'r');
+    EXPECT_TRUE(std::all_of(angels.begin(), angels.end(), [](char c) { return c == 'r'; }));
+}
+
+TEST(dyn_array_tests, use_std_algorithms)
+{
+    gsl::dyn_array<char> dodgers(26);
+    std::generate(dodgers.begin(), dodgers.end(), [i = 0]() mutable { return 'a' + i++; });
+    char ch = 'a';
+    for (auto x : dodgers) EXPECT_EQ(x, ch++);
+    EXPECT_EQ(std::find(dodgers.begin(), dodgers.end(), 'a'), dodgers.begin());
+    {
+        auto it = std::find(dodgers.begin(), dodgers.end(), 'c');
+        EXPECT_EQ(std::distance(dodgers.begin(), it), 'c' - 'a');
+        EXPECT_EQ(std::distance(it, dodgers.begin()), 'a' - 'c');
+    }
+    {
+        auto it = std::lower_bound(dodgers.begin(), dodgers.end(), 'j');
+        EXPECT_EQ(*it, 'j');
+        EXPECT_EQ(std::distance(dodgers.begin(), it), 'j' - 'a');
+        EXPECT_EQ(std::distance(it, dodgers.begin()), 'a' - 'j');
+    }
+    EXPECT_EQ(dodgers.begin(), std::begin(dodgers));
+    EXPECT_EQ(dodgers.end(), std::end(dodgers));
+}
+
+#ifdef GSL_HAS_CONSTEXPR_ALLOCATOR
+TEST(dyn_array_tests, constexprness)
+{
+    constexpr gsl::dyn_array<char> marlins;
+    static_assert(marlins == marlins);
+    static_assert(marlins.empty());
+    static_assert(marlins.size() == 0);
+    static_assert(marlins.data() == nullptr);
+    static_assert(marlins.begin() == marlins.end());
+    static_assert(std::distance(marlins.begin(), marlins.end()) == 0);
+}
+#endif /* GSL_HAS_CONSTEXPR_ALLOCATOR */
+
+#ifdef GSL_HAS_RANGES
+TEST(dyn_array_tests, ranges)
+{
+    gsl::dyn_array<char> brewers(26);
+    std::ranges::generate(brewers, [c = 'a']() mutable { return c++; });
+    char ch = 'a';
+    for (auto x : brewers) EXPECT_EQ(x, ch++);
+    EXPECT_EQ(std::ranges::find(brewers, 'a'), std::ranges::begin(brewers));
+    {
+        auto it = std::ranges::find(brewers, 'c');
+        EXPECT_EQ(std::ranges::distance(std::ranges::begin(brewers), it), 'c' - 'a');
+        EXPECT_EQ(std::ranges::distance(it, std::ranges::begin(brewers)), 'a' - 'c');
+    }
+
+#ifdef GSL_HAS_CONTAINER_RANGES
+    std::vector<char> twins(10, 'c');
+    gsl::dyn_array<char> mets(std::from_range_t{}, twins);
+    EXPECT_EQ(twins.size(), mets.size());
+    EXPECT_TRUE(std::ranges::all_of(mets, [](char c) { return c == 'c'; }));
+#endif /* GSL_HAS_CONTAINER_RANGES */
+}
+#endif /* GSL_HAS_RANGES */
+
+#ifdef GSL_HAS_CONSTEXPR_ALLOCATOR
+template <typename T, unsigned N>
+struct ConstexprAllocator
+{
+    T buf[N];
+    std::size_t sz;
+
+    constexpr ConstexprAllocator() : buf{}, sz{} {}
+
+    constexpr auto allocate(std::size_t n)
+    {
+        auto addr = &buf[sz];
+        sz += n;
+        return addr;
+    }
+
+    constexpr void deallocate(T*, size_t) {}
+};
+#endif /* GSL_HAS_CONSTEXPR_ALLOCATOR */
+
+template <typename T>
+static int AllocCounter = 0;
+
+template <typename T>
+static int DeallocCounter = 0;
+
+template <typename T>
+class Newocator
+{
+    static int allocations;
+    static int deallocations;
+
+public:
+    static void init()
+    {
+        AllocCounter<Newocator<T>> = 0;
+        DeallocCounter<Newocator<T>> = 0;
+    }
+
+    static void check() { EXPECT_EQ(AllocCounter<Newocator<T>>, DeallocCounter<Newocator<T>>); }
+
+    T* allocate(std::size_t n)
+    {
+        AllocCounter<Newocator<T>> ++;
+        return static_cast<T*>(new T[n]);
+    }
+
+    void deallocate(T* p, size_t)
+    {
+        DeallocCounter<Newocator<T>> ++;
+        delete[] p;
+    }
+};
+
+TEST(dyn_array_tests, custom_allocator)
+{
+#ifdef GSL_HAS_CONSTEXPR_ALLOCATOR
+    static constexpr gsl::dyn_array<char, ConstexprAllocator<char, 10>> mets(10, 'c');
+    static_assert(mets.size() == 10);
+    static_assert(mets[0] == 'c');
+    static_assert(std::all_of(std::begin(mets), std::end(mets), [](char c) { return c == 'c'; }));
+#endif /* GSL_HAS_CONSTEXPR_ALLOCATOR */
+
+    Newocator<char>::init();
+    {
+        gsl::dyn_array<char, Newocator<char>> yankees(10, 'c');
+        EXPECT_EQ(yankees.size(), 10);
+        EXPECT_TRUE(
+            std::all_of(std::begin(yankees), std::end(yankees), [](char c) { return c == 'c'; }));
+        yankees[0] = 'a';
+        yankees[1] = 'b';
+        EXPECT_EQ(yankees[0], 'a');
+        EXPECT_EQ(yankees[1], 'b');
+        EXPECT_EQ(yankees[2], 'c');
+        yankees.get_allocator().deallocate(yankees.get_allocator().allocate(1), 1);
+    }
+    Newocator<char>::check();
+}
+
+TEST(dyn_array_tests, init_list)
+{
+    gsl::dyn_array<char> phillies = {'a', 'b', 'c'};
+    EXPECT_EQ(phillies.size(), 3);
+    EXPECT_EQ(phillies[0], 'a');
+    EXPECT_EQ(phillies[1], 'b');
+    EXPECT_EQ(phillies[2], 'c');
+}
+
+TEST(dyn_array_tests, const_operations)
+{
+    const gsl::dyn_array<char> pirates{'a', 'b', 'c', 'd'};
+    EXPECT_EQ(pirates.size(), 4);
+    EXPECT_EQ(pirates[0], 'a');
+}
+
+TEST(dyn_array_tests, reverse_iterator)
+{
+    const gsl::dyn_array<char> padres{'a', 'b', 'c'};
+    auto it = std::rbegin(padres);
+    EXPECT_EQ(*it++, 'c');
+    EXPECT_EQ(*it++, 'b');
+    EXPECT_EQ(*it++, 'a');
+}
+
+#ifdef GSL_HAS_DEDUCTION_GUIDES
+TEST(dyn_array_tests, deduction_guides)
+{
+    std::vector<char> giants{10};
+#ifdef GSL_HAS_CONTAINER_RANGES
+    gsl::dyn_array mariners(std::from_range_t{}, giants);
+#endif /* GSL_HAS_CONTAINER_RANGES */
+    gsl::dyn_array cardinals(std::begin(giants), std::end(giants));
+}
+#endif /* GSL_HAS_DEDUCTION_GUIDES */
diff --git a/tests/span_tests.cpp b/tests/span_tests.cpp
index 9b74ad44..aad88c53 100644
--- a/tests/span_tests.cpp
+++ b/tests/span_tests.cpp
@@ -32,14 +32,14 @@
 #include <vector> // for vector
 
 // the string_view include and macro are used in the deduction guide verification
-#if (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L))
+#ifdef GSL_HAS_DEDUCTION_GUIDES
 #ifdef __has_include
 #if __has_include(<string_view>)
 #include <string_view>
 #define HAS_STRING_VIEW
 #endif // __has_include(<string_view>)
 #endif // __has_include
-#endif // (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L))
+#endif // GSL_HAS_DEDUCTION_GUIDES
 #if defined(__cplusplus) && __cplusplus >= 202002L
 #include <span>
 #endif // __cplusplus >= 202002L
@@ -1209,7 +1209,7 @@ TEST(span_test, default_constructible)
 
 TEST(span_test, std_container_ctad)
 {
-#if (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L))
+#ifdef GSL_HAS_DEDUCTION_GUIDES
     // this test is just to verify that these compile
     {
         std::vector<int> v{1, 2, 3, 4};
@@ -1228,7 +1228,7 @@ TEST(span_test, std_container_ctad)
         static_assert(std::is_same<decltype(sp), gsl::span<const char>>::value);
     }
 #endif
-#endif
+#endif // GSL_HAS_DEDUCTION_GUIDES
 }
 
 TEST(span_test, front_back)
@@ -1294,13 +1294,13 @@ TEST(span_test, conversions)
 {
     int arr[5] = {1, 2, 3, 4, 5};
 
-#if defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L)
+#ifdef GSL_HAS_DEDUCTION_GUIDES
     span s = arr;
     span cs = s;
-#else
+#else // ^^^ deduction guides /// no deduction guides vvv
     span<int, 5> s = arr;
     span<int, 5> cs = s;
-#endif
+#endif // GSL_HAS_DEDUCTION_GUIDES
 
     EXPECT_TRUE(cs.size() == s.size());
     EXPECT_TRUE(cs.data() == s.data());