diff --git a/build-aux/m4/ax_cxx_compile_stdcxx.m4 b/build-aux/m4/ax_cxx_compile_stdcxx.m4
index f147cee3b1..43087b2e68 100644
--- a/build-aux/m4/ax_cxx_compile_stdcxx.m4
+++ b/build-aux/m4/ax_cxx_compile_stdcxx.m4
@@ -1,5 +1,5 @@
 # ===========================================================================
-#   http://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html
+#  https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html
 # ===========================================================================
 #
 # SYNOPSIS
@@ -33,21 +33,23 @@
 #   Copyright (c) 2014, 2015 Google Inc.; contributed by Alexey Sokolov <sokolov@google.com>
 #   Copyright (c) 2015 Paul Norman <penorman@mac.com>
 #   Copyright (c) 2015 Moritz Klammler <moritz@klammler.eu>
+#   Copyright (c) 2016, 2018 Krzesimir Nowak <qdlacz@gmail.com>
+#   Copyright (c) 2019 Enji Cooper <yaneurabeya@gmail.com>
 #
 #   Copying and distribution of this file, with or without modification, are
 #   permitted in any medium without royalty provided the copyright notice
 #   and this notice are preserved.  This file is offered as-is, without any
 #   warranty.
 
-#serial 4
+#serial 11
 
 dnl  This macro is based on the code from the AX_CXX_COMPILE_STDCXX_11 macro
 dnl  (serial version number 13).
 
 AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
-  m4_if([$1], [11], [],
-        [$1], [14], [],
-        [$1], [17], [m4_fatal([support for C++17 not yet implemented in AX_CXX_COMPILE_STDCXX])],
+  m4_if([$1], [11], [ax_cxx_compile_alternatives="11 0x"],
+        [$1], [14], [ax_cxx_compile_alternatives="14 1y"],
+        [$1], [17], [ax_cxx_compile_alternatives="17 1z"],
         [m4_fatal([invalid first argument `$1' to AX_CXX_COMPILE_STDCXX])])dnl
   m4_if([$2], [], [],
         [$2], [ext], [],
@@ -57,26 +59,13 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
         [$3], [mandatory], [ax_cxx_compile_cxx$1_required=true],
         [$3], [optional], [ax_cxx_compile_cxx$1_required=false],
         [m4_fatal([invalid third argument `$3' to AX_CXX_COMPILE_STDCXX])])
-  m4_if([$4], [], [ax_cxx_compile_cxx$1_try_default=true],
-        [$4], [default], [ax_cxx_compile_cxx$1_try_default=true],
-        [$4], [nodefault], [ax_cxx_compile_cxx$1_try_default=false],
-        [m4_fatal([invalid fourth argument `$4' to AX_CXX_COMPILE_STDCXX])])
   AC_LANG_PUSH([C++])dnl
   ac_success=no
 
-  m4_if([$4], [nodefault], [], [dnl
-  AC_CACHE_CHECK(whether $CXX supports C++$1 features by default,
-  ax_cv_cxx_compile_cxx$1,
-  [AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
-    [ax_cv_cxx_compile_cxx$1=yes],
-    [ax_cv_cxx_compile_cxx$1=no])])
-  if test x$ax_cv_cxx_compile_cxx$1 = xyes; then
-    ac_success=yes
-  fi])
-
   m4_if([$2], [noext], [], [dnl
   if test x$ac_success = xno; then
-    for switch in -std=gnu++$1 -std=gnu++0x; do
+    for alternative in ${ax_cxx_compile_alternatives}; do
+      switch="-std=gnu++${alternative}"
       cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
       AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
                      $cachevar,
@@ -102,22 +91,27 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
     dnl HP's aCC needs +std=c++11 according to:
     dnl http://h21007.www2.hp.com/portal/download/files/unprot/aCxx/PDF_Release_Notes/769149-001.pdf
     dnl Cray's crayCC needs "-h std=c++11"
-    for switch in -std=c++$1 -std=c++0x +std=c++$1 "-h std=c++$1"; do
-      cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
-      AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
-                     $cachevar,
-        [ac_save_CXX="$CXX"
-         CXX="$CXX $switch"
-         AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
-          [eval $cachevar=yes],
-          [eval $cachevar=no])
-         CXX="$ac_save_CXX"])
-      if eval test x\$$cachevar = xyes; then
-        CXX="$CXX $switch"
-        if test -n "$CXXCPP" ; then
-          CXXCPP="$CXXCPP $switch"
+    for alternative in ${ax_cxx_compile_alternatives}; do
+      for switch in -std=c++${alternative} +std=c++${alternative} "-h std=c++${alternative}"; do
+        cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
+        AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
+                       $cachevar,
+          [ac_save_CXX="$CXX"
+           CXX="$CXX $switch"
+           AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
+            [eval $cachevar=yes],
+            [eval $cachevar=no])
+           CXX="$ac_save_CXX"])
+        if eval test x\$$cachevar = xyes; then
+          CXX="$CXX $switch"
+          if test -n "$CXXCPP" ; then
+            CXXCPP="$CXXCPP $switch"
+          fi
+          ac_success=yes
+          break
         fi
-        ac_success=yes
+      done
+      if test x$ac_success = xyes; then
         break
       fi
     done
@@ -154,6 +148,11 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_14],
   _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
 )
 
+m4_define([_AX_CXX_COMPILE_STDCXX_testbody_17],
+  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
+  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
+  _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
+)
 
 dnl  Tests for new features in C++11
 
@@ -191,11 +190,13 @@ namespace cxx11
 
     struct Base
     {
+      virtual ~Base() {}
       virtual void f() {}
     };
 
     struct Derived : public Base
     {
+      virtual ~Derived() override {}
       virtual void f() override {}
     };
 
@@ -524,7 +525,7 @@ namespace cxx14
 
   }
 
-  namespace test_digit_seperators
+  namespace test_digit_separators
   {
 
     constexpr auto ten_million = 100'000'000;
@@ -566,3 +567,385 @@ namespace cxx14
 #endif  // __cplusplus >= 201402L
 
 ]])
+
+
+dnl  Tests for new features in C++17
+
+m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_17], [[
+
+// If the compiler admits that it is not ready for C++17, why torture it?
+// Hopefully, this will speed up the test.
+
+#ifndef __cplusplus
+
+#error "This is not a C++ compiler"
+
+#elif __cplusplus < 201703L
+
+#error "This is not a C++17 compiler"
+
+#else
+
+#include <initializer_list>
+#include <utility>
+#include <type_traits>
+
+namespace cxx17
+{
+
+  namespace test_constexpr_lambdas
+  {
+
+    constexpr int foo = [](){return 42;}();
+
+  }
+
+  namespace test::nested_namespace::definitions
+  {
+
+  }
+
+  namespace test_fold_expression
+  {
+
+    template<typename... Args>
+    int multiply(Args... args)
+    {
+      return (args * ... * 1);
+    }
+
+    template<typename... Args>
+    bool all(Args... args)
+    {
+      return (args && ...);
+    }
+
+  }
+
+  namespace test_extended_static_assert
+  {
+
+    static_assert (true);
+
+  }
+
+  namespace test_auto_brace_init_list
+  {
+
+    auto foo = {5};
+    auto bar {5};
+
+    static_assert(std::is_same<std::initializer_list<int>, decltype(foo)>::value);
+    static_assert(std::is_same<int, decltype(bar)>::value);
+  }
+
+  namespace test_typename_in_template_template_parameter
+  {
+
+    template<template<typename> typename X> struct D;
+
+  }
+
+  namespace test_fallthrough_nodiscard_maybe_unused_attributes
+  {
+
+    int f1()
+    {
+      return 42;
+    }
+
+    [[nodiscard]] int f2()
+    {
+      [[maybe_unused]] auto unused = f1();
+
+      switch (f1())
+      {
+      case 17:
+        f1();
+        [[fallthrough]];
+      case 42:
+        f1();
+      }
+      return f1();
+    }
+
+  }
+
+  namespace test_extended_aggregate_initialization
+  {
+
+    struct base1
+    {
+      int b1, b2 = 42;
+    };
+
+    struct base2
+    {
+      base2() {
+        b3 = 42;
+      }
+      int b3;
+    };
+
+    struct derived : base1, base2
+    {
+        int d;
+    };
+
+    derived d1 {{1, 2}, {}, 4};  // full initialization
+    derived d2 {{}, {}, 4};      // value-initialized bases
+
+  }
+
+  namespace test_general_range_based_for_loop
+  {
+
+    struct iter
+    {
+      int i;
+
+      int& operator* ()
+      {
+        return i;
+      }
+
+      const int& operator* () const
+      {
+        return i;
+      }
+
+      iter& operator++()
+      {
+        ++i;
+        return *this;
+      }
+    };
+
+    struct sentinel
+    {
+      int i;
+    };
+
+    bool operator== (const iter& i, const sentinel& s)
+    {
+      return i.i == s.i;
+    }
+
+    bool operator!= (const iter& i, const sentinel& s)
+    {
+      return !(i == s);
+    }
+
+    struct range
+    {
+      iter begin() const
+      {
+        return {0};
+      }
+
+      sentinel end() const
+      {
+        return {5};
+      }
+    };
+
+    void f()
+    {
+      range r {};
+
+      for (auto i : r)
+      {
+        [[maybe_unused]] auto v = i;
+      }
+    }
+
+  }
+
+  namespace test_lambda_capture_asterisk_this_by_value
+  {
+
+    struct t
+    {
+      int i;
+      int foo()
+      {
+        return [*this]()
+        {
+          return i;
+        }();
+      }
+    };
+
+  }
+
+  namespace test_enum_class_construction
+  {
+
+    enum class byte : unsigned char
+    {};
+
+    byte foo {42};
+
+  }
+
+  namespace test_constexpr_if
+  {
+
+    template <bool cond>
+    int f ()
+    {
+      if constexpr(cond)
+      {
+        return 13;
+      }
+      else
+      {
+        return 42;
+      }
+    }
+
+  }
+
+  namespace test_selection_statement_with_initializer
+  {
+
+    int f()
+    {
+      return 13;
+    }
+
+    int f2()
+    {
+      if (auto i = f(); i > 0)
+      {
+        return 3;
+      }
+
+      switch (auto i = f(); i + 4)
+      {
+      case 17:
+        return 2;
+
+      default:
+        return 1;
+      }
+    }
+
+  }
+
+  namespace test_template_argument_deduction_for_class_templates
+  {
+
+    template <typename T1, typename T2>
+    struct pair
+    {
+      pair (T1 p1, T2 p2)
+        : m1 {p1},
+          m2 {p2}
+      {}
+
+      T1 m1;
+      T2 m2;
+    };
+
+    void f()
+    {
+      [[maybe_unused]] auto p = pair{13, 42u};
+    }
+
+  }
+
+  namespace test_non_type_auto_template_parameters
+  {
+
+    template <auto n>
+    struct B
+    {};
+
+    B<5> b1;
+    B<'a'> b2;
+
+  }
+
+  namespace test_structured_bindings
+  {
+
+    int arr[2] = { 1, 2 };
+    std::pair<int, int> pr = { 1, 2 };
+
+    auto f1() -> int(&)[2]
+    {
+      return arr;
+    }
+
+    auto f2() -> std::pair<int, int>&
+    {
+      return pr;
+    }
+
+    struct S
+    {
+      int x1 : 2;
+      volatile double y1;
+    };
+
+    S f3()
+    {
+      return {};
+    }
+
+    auto [ x1, y1 ] = f1();
+    auto& [ xr1, yr1 ] = f1();
+    auto [ x2, y2 ] = f2();
+    auto& [ xr2, yr2 ] = f2();
+    const auto [ x3, y3 ] = f3();
+
+  }
+
+  namespace test_exception_spec_type_system
+  {
+
+    struct Good {};
+    struct Bad {};
+
+    void g1() noexcept;
+    void g2();
+
+    template<typename T>
+    Bad
+    f(T*, T*);
+
+    template<typename T1, typename T2>
+    Good
+    f(T1*, T2*);
+
+    static_assert (std::is_same_v<Good, decltype(f(g1, g2))>);
+
+  }
+
+  namespace test_inline_variables
+  {
+
+    template<class T> void f(T)
+    {}
+
+    template<class T> inline T g(T)
+    {
+      return T{};
+    }
+
+    template<> inline void f<>(int)
+    {}
+
+    template<> int g<>(int)
+    {
+      return 5;
+    }
+
+  }
+
+}  // namespace cxx17
+
+#endif  // __cplusplus < 201703L
+
+]])
diff --git a/configure.ac b/configure.ac
index 714e7a99c9..46b53eb244 100644
--- a/configure.ac
+++ b/configure.ac
@@ -60,8 +60,20 @@ case $host in
      lt_cv_deplibs_check_method="pass_all"
   ;;
 esac
-dnl Require C++14 compiler (no GNU extensions)
-AX_CXX_COMPILE_STDCXX([14], [noext], [mandatory], [nodefault])
+
+AC_ARG_ENABLE([c++17],
+  [AS_HELP_STRING([--enable-c++17],
+  [enable compilation in c++17 mode (disabled by default)])],
+  [use_cxx17=$enableval],
+  [use_cxx17=no])
+
+dnl Require C++14 or C++17 compiler (no GNU extensions)
+if test "x$use_cxx17" = xyes; then
+  AX_CXX_COMPILE_STDCXX([17], [noext], [mandatory])
+else
+  AX_CXX_COMPILE_STDCXX([14], [noext], [mandatory])
+fi
+
 dnl Check if -latomic is required for <std::atomic>
 CHECK_ATOMIC
 
diff --git a/src/attributes.h b/src/attributes.h
index 45099bd8b8..9d5c1d44a0 100644
--- a/src/attributes.h
+++ b/src/attributes.h
@@ -19,4 +19,14 @@
 #  endif
 #endif
 
+#if defined(__clang__)
+#  if __has_attribute(lifetimebound)
+#    define LIFETIMEBOUND [[clang::lifetimebound]]
+#  else
+#    define LIFETIMEBOUND
+#  endif
+#else
+#  define LIFETIMEBOUND
+#endif
+
 #endif // BITCOIN_ATTRIBUTES_H
diff --git a/src/span.h b/src/span.h
index 707fc21918..88a078e902 100644
--- a/src/span.h
+++ b/src/span.h
@@ -7,6 +7,26 @@
 
 #include <type_traits>
 #include <cstddef>
+#include <algorithm>
+#include <assert.h>
+
+#ifdef DEBUG
+#define CONSTEXPR_IF_NOT_DEBUG
+#define ASSERT_IF_DEBUG(x) assert((x))
+#else
+#define CONSTEXPR_IF_NOT_DEBUG constexpr
+#define ASSERT_IF_DEBUG(x)
+#endif
+
+#if defined(__clang__)
+#if __has_attribute(lifetimebound)
+#define SPAN_ATTR_LIFETIMEBOUND [[clang::lifetimebound]]
+#else
+#define SPAN_ATTR_LIFETIMEBOUND
+#endif
+#else
+#define SPAN_ATTR_LIFETIMEBOUND
+#endif
 
 /** A Span is an object that can refer to a contiguous sequence of objects.
  *
@@ -16,25 +36,147 @@ template<typename C>
 class Span
 {
     C* m_data;
-    std::ptrdiff_t m_size;
+    std::size_t m_size;
+
+    template <class T>
+    struct is_Span_int : public std::false_type {};
+    template <class T>
+    struct is_Span_int<Span<T>> : public std::true_type {};
+    template <class T>
+    struct is_Span : public is_Span_int<typename std::remove_cv<T>::type>{};
+
 
 public:
     constexpr Span() noexcept : m_data(nullptr), m_size(0) {}
-    constexpr Span(C* data, std::ptrdiff_t size) noexcept : m_data(data), m_size(size) {}
+
+    /** Construct a span from a begin pointer and a size.
+     *
+     * This implements a subset of the iterator-based std::span constructor in C++20,
+     * which is hard to implement without std::address_of.
+     */
+    template <typename T, typename std::enable_if<std::is_convertible<T (*)[], C (*)[]>::value, int>::type = 0>
+    constexpr Span(T* begin, std::size_t size) noexcept : m_data(begin), m_size(size) {}
+
+    /** Construct a span from a begin and end pointer.
+     *
+     * This implements a subset of the iterator-based std::span constructor in C++20,
+     * which is hard to implement without std::address_of.
+     */
+    template <typename T, typename std::enable_if<std::is_convertible<T (*)[], C (*)[]>::value, int>::type = 0>
+    CONSTEXPR_IF_NOT_DEBUG Span(T* begin, T* end) noexcept : m_data(begin), m_size(end - begin)
+    {
+        ASSERT_IF_DEBUG(end >= begin);
+    }
+
+    /** Implicit conversion of spans between compatible types.
+     *
+     *  Specifically, if a pointer to an array of type O can be implicitly converted to a pointer to an array of type
+     *  C, then permit implicit conversion of Span<O> to Span<C>. This matches the behavior of the corresponding
+     *  C++20 std::span constructor.
+     *
+     *  For example this means that a Span<T> can be converted into a Span<const T>.
+     */
+    template <typename O, typename std::enable_if<std::is_convertible<O (*)[], C (*)[]>::value, int>::type = 0>
+    constexpr Span(const Span<O>& other) noexcept : m_data(other.m_data), m_size(other.m_size) {}
+
+    /** Default copy constructor. */
+    constexpr Span(const Span&) noexcept = default;
+
+    /** Default assignment operator. */
+    Span& operator=(const Span& other) noexcept = default;
+
+    /** Construct a Span from an array. This matches the corresponding C++20 std::span constructor. */
+    template <int N>
+    constexpr Span(C (&a)[N]) noexcept : m_data(a), m_size(N) {}
+
+    /** Construct a Span for objects with .data() and .size() (std::string, std::array, std::vector, ...).
+     *
+     * This implements a subset of the functionality provided by the C++20 std::span range-based constructor.
+     *
+     * To prevent surprises, only Spans for constant value types are supported when passing in temporaries.
+     * Note that this restriction does not exist when converting arrays or other Spans (see above).
+     */
+    template <typename V>
+    constexpr Span(V& other SPAN_ATTR_LIFETIMEBOUND,
+        typename std::enable_if<!is_Span<V>::value &&
+                                std::is_convertible<typename std::remove_pointer<decltype(std::declval<V&>().data())>::type (*)[], C (*)[]>::value &&
+                                std::is_convertible<decltype(std::declval<V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr)
+        : m_data(other.data()), m_size(other.size()){}
+
+    template <typename V>
+    constexpr Span(const V& other SPAN_ATTR_LIFETIMEBOUND,
+        typename std::enable_if<!is_Span<V>::value &&
+                                std::is_convertible<typename std::remove_pointer<decltype(std::declval<const V&>().data())>::type (*)[], C (*)[]>::value &&
+                                std::is_convertible<decltype(std::declval<const V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr)
+        : m_data(other.data()), m_size(other.size()){}
 
     constexpr C* data() const noexcept { return m_data; }
-    constexpr std::ptrdiff_t size() const noexcept { return m_size; }
+    constexpr C* begin() const noexcept { return m_data; }
+    constexpr C* end() const noexcept { return m_data + m_size; }
+    CONSTEXPR_IF_NOT_DEBUG C& front() const noexcept
+    {
+        ASSERT_IF_DEBUG(size() > 0);
+        return m_data[0];
+    }
+    CONSTEXPR_IF_NOT_DEBUG C& back() const noexcept
+    {
+        ASSERT_IF_DEBUG(size() > 0);
+        return m_data[m_size - 1];
+    }
+    constexpr std::size_t size() const noexcept { return m_size; }
+    CONSTEXPR_IF_NOT_DEBUG C& operator[](std::size_t pos) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() > pos);
+        return m_data[pos];
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> subspan(std::size_t offset) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() >= offset);
+        return Span<C>(m_data + offset, m_size - offset);
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> subspan(std::size_t offset, std::size_t count) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() >= offset + count);
+        return Span<C>(m_data + offset, count);
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> first(std::size_t count) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() >= count);
+        return Span<C>(m_data, count);
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> last(std::size_t count) const noexcept
+    {
+         ASSERT_IF_DEBUG(size() >= count);
+         return Span<C>(m_data + m_size - count, count);
+    }
+
+    friend constexpr bool operator==(const Span& a, const Span& b) noexcept { return a.size() == b.size() && std::equal(a.begin(), a.end(), b.begin()); }
+    friend constexpr bool operator!=(const Span& a, const Span& b) noexcept { return !(a == b); }
+    friend constexpr bool operator<(const Span& a, const Span& b) noexcept { return std::lexicographical_compare(a.begin(), a.end(), b.begin(), b.end()); }
+    friend constexpr bool operator<=(const Span& a, const Span& b) noexcept { return !(b < a); }
+    friend constexpr bool operator>(const Span& a, const Span& b) noexcept { return (b < a); }
+    friend constexpr bool operator>=(const Span& a, const Span& b) noexcept { return !(a < b); }
+
+    template <typename O> friend class Span;
 };
 
-/** Create a span to a container exposing data() and size().
- *
- * This correctly deals with constness: the returned Span's element type will be
- * whatever data() returns a pointer to. If either the passed container is const,
- * or its element type is const, the resulting span will have a const element type.
- *
- * std::span will have a constructor that implements this functionality directly.
- */
-template<typename V>
-constexpr Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type> MakeSpan(V& v) { return Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type>(v.data(), v.size()); }
+// MakeSpan helps constructing a Span of the right type automatically.
+/** MakeSpan for arrays: */
+template <typename A, int N> Span<A> constexpr MakeSpan(A (&a)[N]) { return Span<A>(a, N); }
+/** MakeSpan for temporaries / rvalue references, only supporting const output. */
+template <typename V> constexpr auto MakeSpan(V&& v SPAN_ATTR_LIFETIMEBOUND) -> typename std::enable_if<!std::is_lvalue_reference<V>::value, Span<const typename std::remove_pointer<decltype(v.data())>::type>>::type { return std::forward<V>(v); }
+/** MakeSpan for (lvalue) references, supporting mutable output. */
+template <typename V> constexpr auto MakeSpan(V& v SPAN_ATTR_LIFETIMEBOUND) -> Span<typename std::remove_pointer<decltype(v.data())>::type> { return v; }
+
+/** Pop the last element off a span, and return a reference to that element. */
+template <typename T>
+T& SpanPopBack(Span<T>& span)
+{
+    size_t size = span.size();
+    ASSERT_IF_DEBUG(size > 0);
+    T& back = span[size - 1];
+    span = Span<T>(span.data(), size - 1);
+    return back;
+}
 
 #endif