diff --git a/src/3rdparty/optional/LICENSE_1_0.txt b/src/3rdparty/optional/LICENSE_1_0.txt
deleted file mode 100644
--- a/src/3rdparty/optional/LICENSE_1_0.txt
+++ /dev/null
@@ -1,23 +0,0 @@
-Boost Software License - Version 1.0 - August 17th, 2003
-
-Permission is hereby granted, free of charge, to any person or organization
-obtaining a copy of the software and accompanying documentation covered by
-this license (the "Software") to use, reproduce, display, distribute,
-execute, and transmit the Software, and to prepare derivative works of the
-Software, and to permit third-parties to whom the Software is furnished to
-do so, all subject to the following:
-
-The copyright notices in the Software and this entire statement, including
-the above license grant, this restriction and the following disclaimer,
-must be included in all copies of the Software, in whole or in part, and
-all derivative works of the Software, unless such copies or derivative
-works are solely in the form of machine-executable object code generated by
-a source language processor.
-
-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, TITLE AND NON-INFRINGEMENT. IN NO EVENT
-SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
-FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
-ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
-DEALINGS IN THE SOFTWARE.
diff --git a/src/3rdparty/optional/optional.hpp b/src/3rdparty/optional/optional.hpp
deleted file mode 100644
--- a/src/3rdparty/optional/optional.hpp
+++ /dev/null
@@ -1,1066 +0,0 @@
-// Copyright (C) 2011 - 2012 Andrzej Krzemienski.
-//
-// Use, modification, and distribution is subject to the Boost Software
-// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
-// http://www.boost.org/LICENSE_1_0.txt)
-//
-// The idea and interface is based on Boost.Optional library
-// authored by Fernando Luis Cacciola Carballal
-
-# ifndef ___OPTIONAL_HPP___
-# define ___OPTIONAL_HPP___
-
-# include <utility>
-# include <type_traits>
-# include <initializer_list>
-# include <cassert>
-# include <functional>
-# include <string>
-# include <stdexcept>
-
-# define TR2_OPTIONAL_REQUIRES(...) typename enable_if<__VA_ARGS__::value, bool>::type = false
-
-# if defined __GNUC__ // NOTE: GNUC is also defined for Clang
-#   if (__GNUC__ == 4) && (__GNUC_MINOR__ >= 8)
-#     define TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
-#   elif (__GNUC__ > 4)
-#     define TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
-#   endif
-#
-#   if (__GNUC__ == 4) && (__GNUC_MINOR__ >= 7)
-#     define TR2_OPTIONAL_GCC_4_7_AND_HIGHER___
-#   elif (__GNUC__ > 4)
-#     define TR2_OPTIONAL_GCC_4_7_AND_HIGHER___
-#   endif
-#
-#   if (__GNUC__ == 4) && (__GNUC_MINOR__ == 8) && (__GNUC_PATCHLEVEL__ >= 1)
-#     define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
-#   elif (__GNUC__ == 4) && (__GNUC_MINOR__ >= 9)
-#     define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
-#   elif (__GNUC__ > 4)
-#     define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
-#   endif
-# endif
-#
-# if defined __clang_major__
-#   if (__clang_major__ == 3 && __clang_minor__ >= 5)
-#     define TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
-#   elif (__clang_major__ > 3)
-#     define TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
-#   endif
-#   if defined TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
-#     define TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
-#   elif (__clang_major__ == 3 && __clang_minor__ == 4 && __clang_patchlevel__ >= 2)
-#     define TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
-#   endif
-# endif
-#
-# if defined _MSC_VER
-#   if (_MSC_VER >= 1900)
-#     define TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
-#   endif
-# endif
-
-# if defined __clang__
-#   if (__clang_major__ > 2) || (__clang_major__ == 2) && (__clang_minor__ >= 9)
-#     define OPTIONAL_HAS_THIS_RVALUE_REFS 1
-#   else
-#     define OPTIONAL_HAS_THIS_RVALUE_REFS 0
-#   endif
-# elif defined TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
-#   define OPTIONAL_HAS_THIS_RVALUE_REFS 1
-# elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
-#   define OPTIONAL_HAS_THIS_RVALUE_REFS 1
-# else
-#   define OPTIONAL_HAS_THIS_RVALUE_REFS 0
-# endif
-
-
-# if defined TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
-#   define OPTIONAL_HAS_CONSTEXPR_INIT_LIST 1
-#   define OPTIONAL_CONSTEXPR_INIT_LIST constexpr
-# else
-#   define OPTIONAL_HAS_CONSTEXPR_INIT_LIST 0
-#   define OPTIONAL_CONSTEXPR_INIT_LIST
-# endif
-
-# if defined TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_ && (defined __cplusplus) && (__cplusplus != 201103L)
-#   define OPTIONAL_HAS_MOVE_ACCESSORS 1
-# else
-#   define OPTIONAL_HAS_MOVE_ACCESSORS 0
-# endif
-
-# // In C++11 constexpr implies const, so we need to make non-const members also non-constexpr
-# if (defined __cplusplus) && (__cplusplus == 201103L)
-#   define OPTIONAL_MUTABLE_CONSTEXPR
-# else
-#   define OPTIONAL_MUTABLE_CONSTEXPR constexpr
-# endif
-
-namespace std{
-
-namespace experimental{
-
-// BEGIN workaround for missing is_trivially_destructible
-# if defined TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
-    // leave it: it is already there
-# elif defined TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
-    // leave it: it is already there
-# elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
-    // leave it: it is already there
-# elif defined TR2_OPTIONAL_DISABLE_EMULATION_OF_TYPE_TRAITS
-    // leave it: the user doesn't want it
-# else
-	template <typename T>
-	using is_trivially_destructible = std::has_trivial_destructor<T>;
-# endif
-// END workaround for missing is_trivially_destructible
-
-# if (defined TR2_OPTIONAL_GCC_4_7_AND_HIGHER___)
-    // leave it; our metafunctions are already defined.
-# elif defined TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
-    // leave it; our metafunctions are already defined.
-# elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
-    // leave it: it is already there
-# elif defined TR2_OPTIONAL_DISABLE_EMULATION_OF_TYPE_TRAITS
-    // leave it: the user doesn't want it
-# else
-
-
-// workaround for missing traits in GCC and CLANG
-template <class T>
-struct is_nothrow_move_constructible
-{
-  constexpr static bool value = std::is_nothrow_constructible<T, T&&>::value;
-};
-
-
-template <class T, class U>
-struct is_assignable
-{
-  template <class X, class Y>
-  constexpr static bool has_assign(...) { return false; }
-
-  template <class X, class Y, size_t S = sizeof((std::declval<X>() = std::declval<Y>(), true)) >
-  // the comma operator is necessary for the cases where operator= returns void
-  constexpr static bool has_assign(bool) { return true; }
-
-  constexpr static bool value = has_assign<T, U>(true);
-};
-
-
-template <class T>
-struct is_nothrow_move_assignable
-{
-  template <class X, bool has_any_move_assign>
-  struct has_nothrow_move_assign {
-    constexpr static bool value = false;
-  };
-
-  template <class X>
-  struct has_nothrow_move_assign<X, true> {
-    constexpr static bool value = noexcept( std::declval<X&>() = std::declval<X&&>() );
-  };
-
-  constexpr static bool value = has_nothrow_move_assign<T, is_assignable<T&, T&&>::value>::value;
-};
-// end workaround
-
-
-# endif
-
-
-
-// 20.5.4, optional for object types
-template <class T> class optional;
-
-// 20.5.5, optional for lvalue reference types
-template <class T> class optional<T&>;
-
-
-// workaround: std utility functions aren't constexpr yet
-template <class T> inline constexpr T&& constexpr_forward(typename std::remove_reference<T>::type& t) noexcept
-{
-  return static_cast<T&&>(t);
-}
-
-template <class T> inline constexpr T&& constexpr_forward(typename std::remove_reference<T>::type&& t) noexcept
-{
-    static_assert(!std::is_lvalue_reference<T>::value, "!!");
-    return static_cast<T&&>(t);
-}
-
-template <class T> inline constexpr typename std::remove_reference<T>::type&& constexpr_move(T&& t) noexcept
-{
-    return static_cast<typename std::remove_reference<T>::type&&>(t);
-}
-
-
-#if defined NDEBUG
-# define TR2_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) (EXPR)
-#else
-# define TR2_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) ((CHECK) ? (EXPR) : ([]{assert(!#CHECK);}(), (EXPR)))
-#endif
-
-
-namespace detail_
-{
-
-// static_addressof: a constexpr version of addressof
-template <typename T>
-struct has_overloaded_addressof
-{
-  template <class X>
-  constexpr static bool has_overload(...) { return false; }
-
-  template <class X, size_t S = sizeof(std::declval<X&>().operator&()) >
-  constexpr static bool has_overload(bool) { return true; }
-
-  constexpr static bool value = has_overload<T>(true);
-};
-
-template <typename T, TR2_OPTIONAL_REQUIRES(!has_overloaded_addressof<T>)>
-constexpr T* static_addressof(T& ref)
-{
-  return &ref;
-}
-
-template <typename T, TR2_OPTIONAL_REQUIRES(has_overloaded_addressof<T>)>
-T* static_addressof(T& ref)
-{
-  return std::addressof(ref);
-}
-
-
-// the call to convert<A>(b) has return type A and converts b to type A iff b decltype(b) is implicitly convertible to A
-template <class U>
-constexpr U convert(U v) { return v; }
-
-
-namespace swap_ns
-{
-  using std::swap;
-
-  template <class T>
-  void adl_swap(T& t, T& u) noexcept(noexcept(swap(t, u)))
-  {
-    swap(t, u);
-  }
-
-} // namespace swap_ns
-
-} // namespace detail
-
-
-constexpr struct trivial_init_t{} trivial_init{};
-
-
-// 20.5.6, In-place construction
-constexpr struct in_place_t{} in_place{};
-
-
-// 20.5.7, Disengaged state indicator
-struct nullopt_t
-{
-  struct init{};
-  constexpr explicit nullopt_t(init){}
-};
-constexpr nullopt_t nullopt{nullopt_t::init()};
-
-
-// 20.5.8, class bad_optional_access
-class bad_optional_access : public logic_error {
-public:
-  explicit bad_optional_access(const string& what_arg) : logic_error{what_arg} {}
-  explicit bad_optional_access(const char* what_arg) : logic_error{what_arg} {}
-};
-
-
-template <class T>
-union storage_t
-{
-  unsigned char dummy_;
-  T value_;
-
-  constexpr storage_t( trivial_init_t ) noexcept : dummy_() {};
-
-  template <class... Args>
-  constexpr storage_t( Args&&... args ) : value_(constexpr_forward<Args>(args)...) {}
-
-  ~storage_t(){}
-};
-
-
-template <class T>
-union constexpr_storage_t
-{
-    unsigned char dummy_;
-    T value_;
-
-    constexpr constexpr_storage_t( trivial_init_t ) noexcept : dummy_() {};
-
-    template <class... Args>
-    constexpr constexpr_storage_t( Args&&... args ) : value_(constexpr_forward<Args>(args)...) {}
-
-    ~constexpr_storage_t() = default;
-};
-
-
-template <class T>
-struct optional_base
-{
-    bool init_;
-    storage_t<T> storage_;
-
-    constexpr optional_base() noexcept : init_(false), storage_(trivial_init) {};
-
-    explicit constexpr optional_base(const T& v) : init_(true), storage_(v) {}
-
-    explicit constexpr optional_base(T&& v) : init_(true), storage_(constexpr_move(v)) {}
-
-    template <class... Args> explicit optional_base(in_place_t, Args&&... args)
-        : init_(true), storage_(constexpr_forward<Args>(args)...) {}
-
-    template <class U, class... Args, TR2_OPTIONAL_REQUIRES(is_constructible<T, std::initializer_list<U>>)>
-    explicit optional_base(in_place_t, std::initializer_list<U> il, Args&&... args)
-        : init_(true), storage_(il, std::forward<Args>(args)...) {}
-
-    ~optional_base() { if (init_) storage_.value_.T::~T(); }
-};
-
-
-template <class T>
-struct constexpr_optional_base
-{
-    bool init_;
-    constexpr_storage_t<T> storage_;
-
-    constexpr constexpr_optional_base() noexcept : init_(false), storage_(trivial_init) {};
-
-    explicit constexpr constexpr_optional_base(const T& v) : init_(true), storage_(v) {}
-
-    explicit constexpr constexpr_optional_base(T&& v) : init_(true), storage_(constexpr_move(v)) {}
-
-    template <class... Args> explicit constexpr constexpr_optional_base(in_place_t, Args&&... args)
-      : init_(true), storage_(constexpr_forward<Args>(args)...) {}
-
-    template <class U, class... Args, TR2_OPTIONAL_REQUIRES(is_constructible<T, std::initializer_list<U>>)>
-    OPTIONAL_CONSTEXPR_INIT_LIST explicit constexpr_optional_base(in_place_t, std::initializer_list<U> il, Args&&... args)
-      : init_(true), storage_(il, std::forward<Args>(args)...) {}
-
-    ~constexpr_optional_base() = default;
-};
-
-template <class T>
-using OptionalBase = typename std::conditional<
-    is_trivially_destructible<T>::value,                          // if possible
-    constexpr_optional_base<typename std::remove_const<T>::type>, // use base with trivial destructor
-    optional_base<typename std::remove_const<T>::type>
->::type;
-
-
-
-template <class T>
-class optional : private OptionalBase<T>
-{
-  static_assert( !std::is_same<typename std::decay<T>::type, nullopt_t>::value, "bad T" );
-  static_assert( !std::is_same<typename std::decay<T>::type, in_place_t>::value, "bad T" );
-
-
-  constexpr bool initialized() const noexcept { return OptionalBase<T>::init_; }
-  typename std::remove_const<T>::type* dataptr() {  return std::addressof(OptionalBase<T>::storage_.value_); }
-  constexpr const T* dataptr() const { return detail_::static_addressof(OptionalBase<T>::storage_.value_); }
-
-# if OPTIONAL_HAS_THIS_RVALUE_REFS == 1
-  constexpr const T& contained_val() const& { return OptionalBase<T>::storage_.value_; }
-#   if OPTIONAL_HAS_MOVE_ACCESSORS == 1
-  OPTIONAL_MUTABLE_CONSTEXPR T&& contained_val() && { return std::move(OptionalBase<T>::storage_.value_); }
-  OPTIONAL_MUTABLE_CONSTEXPR T& contained_val() & { return OptionalBase<T>::storage_.value_; }
-#   else
-  T& contained_val() & { return OptionalBase<T>::storage_.value_; }
-  T&& contained_val() && { return std::move(OptionalBase<T>::storage_.value_); }
-#   endif
-# else
-  constexpr const T& contained_val() const { return OptionalBase<T>::storage_.value_; }
-  T& contained_val() { return OptionalBase<T>::storage_.value_; }
-# endif
-
-  void clear() noexcept {
-    if (initialized()) dataptr()->T::~T();
-    OptionalBase<T>::init_ = false;
-  }
-
-  template <class... Args>
-  void initialize(Args&&... args) noexcept(noexcept(T(std::forward<Args>(args)...)))
-  {
-    assert(!OptionalBase<T>::init_);
-    ::new (static_cast<void*>(dataptr())) T(std::forward<Args>(args)...);
-    OptionalBase<T>::init_ = true;
-  }
-
-  template <class U, class... Args>
-  void initialize(std::initializer_list<U> il, Args&&... args) noexcept(noexcept(T(il, std::forward<Args>(args)...)))
-  {
-    assert(!OptionalBase<T>::init_);
-    ::new (static_cast<void*>(dataptr())) T(il, std::forward<Args>(args)...);
-    OptionalBase<T>::init_ = true;
-  }
-
-public:
-  typedef T value_type;
-
-  // 20.5.5.1, constructors
-  constexpr optional() noexcept : OptionalBase<T>()  {};
-  constexpr optional(nullopt_t) noexcept : OptionalBase<T>() {};
-
-  optional(const optional& rhs)
-  : OptionalBase<T>()
-  {
-    if (rhs.initialized()) {
-        ::new (static_cast<void*>(dataptr())) T(*rhs);
-        OptionalBase<T>::init_ = true;
-    }
-  }
-
-  optional(optional&& rhs) noexcept(is_nothrow_move_constructible<T>::value)
-  : OptionalBase<T>()
-  {
-    if (rhs.initialized()) {
-        ::new (static_cast<void*>(dataptr())) T(std::move(*rhs));
-        OptionalBase<T>::init_ = true;
-    }
-  }
-
-  constexpr optional(const T& v) : OptionalBase<T>(v) {}
-
-  constexpr optional(T&& v) : OptionalBase<T>(constexpr_move(v)) {}
-
-  template <class... Args>
-  explicit constexpr optional(in_place_t, Args&&... args)
-  : OptionalBase<T>(in_place_t{}, constexpr_forward<Args>(args)...) {}
-
-  template <class U, class... Args, TR2_OPTIONAL_REQUIRES(is_constructible<T, std::initializer_list<U>>)>
-  OPTIONAL_CONSTEXPR_INIT_LIST explicit optional(in_place_t, std::initializer_list<U> il, Args&&... args)
-  : OptionalBase<T>(in_place_t{}, il, constexpr_forward<Args>(args)...) {}
-
-  // 20.5.4.2, Destructor
-  ~optional() = default;
-
-  // 20.5.4.3, assignment
-  optional& operator=(nullopt_t) noexcept
-  {
-    clear();
-    return *this;
-  }
-
-  optional& operator=(const optional& rhs)
-  {
-    if      (initialized() == true  && rhs.initialized() == false) clear();
-    else if (initialized() == false && rhs.initialized() == true)  initialize(*rhs);
-    else if (initialized() == true  && rhs.initialized() == true)  contained_val() = *rhs;
-    return *this;
-  }
-
-  optional& operator=(optional&& rhs)
-  noexcept(is_nothrow_move_assignable<T>::value && is_nothrow_move_constructible<T>::value)
-  {
-    if      (initialized() == true  && rhs.initialized() == false) clear();
-    else if (initialized() == false && rhs.initialized() == true)  initialize(std::move(*rhs));
-    else if (initialized() == true  && rhs.initialized() == true)  contained_val() = std::move(*rhs);
-    return *this;
-  }
-
-  template <class U>
-  auto operator=(U&& v)
-  -> typename enable_if
-  <
-    is_same<typename decay<U>::type, T>::value,
-    optional&
-  >::type
-  {
-    if (initialized()) { contained_val() = std::forward<U>(v); }
-    else               { initialize(std::forward<U>(v));  }
-    return *this;
-  }
-
-
-  template <class... Args>
-  void emplace(Args&&... args)
-  {
-    clear();
-    initialize(std::forward<Args>(args)...);
-  }
-
-  template <class U, class... Args>
-  void emplace(initializer_list<U> il, Args&&... args)
-  {
-    clear();
-    initialize<U, Args...>(il, std::forward<Args>(args)...);
-  }
-
-  // 20.5.4.4, Swap
-  void swap(optional<T>& rhs) noexcept(is_nothrow_move_constructible<T>::value
-                                       && noexcept(detail_::swap_ns::adl_swap(declval<T&>(), declval<T&>())))
-  {
-    if      (initialized() == true  && rhs.initialized() == false) { rhs.initialize(std::move(**this)); clear(); }
-    else if (initialized() == false && rhs.initialized() == true)  { initialize(std::move(*rhs)); rhs.clear(); }
-    else if (initialized() == true  && rhs.initialized() == true)  { using std::swap; swap(**this, *rhs); }
-  }
-
-  // 20.5.4.5, Observers
-
-  explicit constexpr operator bool() const noexcept { return initialized(); }
-  constexpr bool has_value() const noexcept { return initialized(); }
-
-  constexpr T const* operator ->() const {
-    return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), dataptr());
-  }
-
-# if OPTIONAL_HAS_MOVE_ACCESSORS == 1
-
-  OPTIONAL_MUTABLE_CONSTEXPR T* operator ->() {
-    assert (initialized());
-    return dataptr();
-  }
-
-  constexpr T const& operator *() const& {
-    return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), contained_val());
-  }
-
-  OPTIONAL_MUTABLE_CONSTEXPR T& operator *() & {
-    assert (initialized());
-    return contained_val();
-  }
-
-  OPTIONAL_MUTABLE_CONSTEXPR T&& operator *() && {
-    assert (initialized());
-    return constexpr_move(contained_val());
-  }
-
-  constexpr T const& value() const& {
-    return initialized() ? contained_val() : (throw bad_optional_access("bad optional access"), contained_val());
-  }
-
-  OPTIONAL_MUTABLE_CONSTEXPR T& value() & {
-    return initialized() ? contained_val() : (throw bad_optional_access("bad optional access"), contained_val());
-  }
-
-  OPTIONAL_MUTABLE_CONSTEXPR T&& value() && {
-    if (!initialized()) throw bad_optional_access("bad optional access");
-    return std::move(contained_val());
-  }
-
-# else
-
-  T* operator ->() {
-    assert (initialized());
-    return dataptr();
-  }
-
-  constexpr T const& operator *() const {
-    return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), contained_val());
-  }
-
-  T& operator *() {
-    assert (initialized());
-    return contained_val();
-  }
-
-  constexpr T const& value() const {
-    return initialized() ? contained_val() : (throw bad_optional_access("bad optional access"), contained_val());
-  }
-
-  T& value() {
-    return initialized() ? contained_val() : (throw bad_optional_access("bad optional access"), contained_val());
-  }
-
-# endif
-
-# if OPTIONAL_HAS_THIS_RVALUE_REFS == 1
-
-  template <class V>
-  constexpr T value_or(V&& v) const&
-  {
-    return *this ? **this : detail_::convert<T>(constexpr_forward<V>(v));
-  }
-
-#   if OPTIONAL_HAS_MOVE_ACCESSORS == 1
-
-  template <class V>
-  OPTIONAL_MUTABLE_CONSTEXPR T value_or(V&& v) &&
-  {
-    return *this ? constexpr_move(const_cast<optional<T>&>(*this).contained_val()) : detail_::convert<T>(constexpr_forward<V>(v));
-  }
-
-#   else
-
-  template <class V>
-  T value_or(V&& v) &&
-  {
-    return *this ? constexpr_move(const_cast<optional<T>&>(*this).contained_val()) : detail_::convert<T>(constexpr_forward<V>(v));
-  }
-
-#   endif
-
-# else
-
-  template <class V>
-  constexpr T value_or(V&& v) const
-  {
-    return *this ? **this : detail_::convert<T>(constexpr_forward<V>(v));
-  }
-
-# endif
-
-  // 20.6.3.6, modifiers
-  void reset() noexcept { clear(); }
-};
-
-
-template <class T>
-class optional<T&>
-{
-  static_assert( !std::is_same<T, nullopt_t>::value, "bad T" );
-  static_assert( !std::is_same<T, in_place_t>::value, "bad T" );
-  T* ref;
-
-public:
-
-  // 20.5.5.1, construction/destruction
-  constexpr optional() noexcept : ref(nullptr) {}
-
-  constexpr optional(nullopt_t) noexcept : ref(nullptr) {}
-
-  constexpr optional(T& v) noexcept : ref(detail_::static_addressof(v)) {}
-
-  optional(T&&) = delete;
-
-  constexpr optional(const optional& rhs) noexcept : ref(rhs.ref) {}
-
-  explicit constexpr optional(in_place_t, T& v) noexcept : ref(detail_::static_addressof(v)) {}
-
-  explicit optional(in_place_t, T&&) = delete;
-
-  ~optional() = default;
-
-  // 20.5.5.2, mutation
-  optional& operator=(nullopt_t) noexcept {
-    ref = nullptr;
-    return *this;
-  }
-
-  // optional& operator=(const optional& rhs) noexcept {
-    // ref = rhs.ref;
-    // return *this;
-  // }
-
-  // optional& operator=(optional&& rhs) noexcept {
-    // ref = rhs.ref;
-    // return *this;
-  // }
-
-  template <typename U>
-  auto operator=(U&& rhs) noexcept
-  -> typename enable_if
-  <
-    is_same<typename decay<U>::type, optional<T&>>::value,
-    optional&
-  >::type
-  {
-    ref = rhs.ref;
-    return *this;
-  }
-
-  template <typename U>
-  auto operator=(U&& rhs) noexcept
-  -> typename enable_if
-  <
-    !is_same<typename decay<U>::type, optional<T&>>::value,
-    optional&
-  >::type
-  = delete;
-
-  void emplace(T& v) noexcept {
-    ref = detail_::static_addressof(v);
-  }
-
-  void emplace(T&&) = delete;
-
-
-  void swap(optional<T&>& rhs) noexcept
-  {
-    std::swap(ref, rhs.ref);
-  }
-
-  // 20.5.5.3, observers
-  constexpr T* operator->() const {
-    return TR2_OPTIONAL_ASSERTED_EXPRESSION(ref, ref);
-  }
-
-  constexpr T& operator*() const {
-    return TR2_OPTIONAL_ASSERTED_EXPRESSION(ref, *ref);
-  }
-
-  constexpr T& value() const {
-    return ref ? *ref : (throw bad_optional_access("bad optional access"), *ref);
-  }
-
-  explicit constexpr operator bool() const noexcept {
-    return ref != nullptr;
-  }
-
-  constexpr bool has_value() const noexcept {
-    return ref != nullptr;
-  }
-
-  template <class V>
-  constexpr typename decay<T>::type value_or(V&& v) const
-  {
-    return *this ? **this : detail_::convert<typename decay<T>::type>(constexpr_forward<V>(v));
-  }
-
-  // x.x.x.x, modifiers
-  void reset() noexcept { ref = nullptr; }
-};
-
-
-template <class T>
-class optional<T&&>
-{
-  static_assert( sizeof(T) == 0, "optional rvalue references disallowed" );
-};
-
-
-// 20.5.8, Relational operators
-template <class T> constexpr bool operator==(const optional<T>& x, const optional<T>& y)
-{
-  return bool(x) != bool(y) ? false : bool(x) == false ? true : *x == *y;
-}
-
-template <class T> constexpr bool operator!=(const optional<T>& x, const optional<T>& y)
-{
-  return !(x == y);
-}
-
-template <class T> constexpr bool operator<(const optional<T>& x, const optional<T>& y)
-{
-  return (!y) ? false : (!x) ? true : *x < *y;
-}
-
-template <class T> constexpr bool operator>(const optional<T>& x, const optional<T>& y)
-{
-  return (y < x);
-}
-
-template <class T> constexpr bool operator<=(const optional<T>& x, const optional<T>& y)
-{
-  return !(y < x);
-}
-
-template <class T> constexpr bool operator>=(const optional<T>& x, const optional<T>& y)
-{
-  return !(x < y);
-}
-
-
-// 20.5.9, Comparison with nullopt
-template <class T> constexpr bool operator==(const optional<T>& x, nullopt_t) noexcept
-{
-  return (!x);
-}
-
-template <class T> constexpr bool operator==(nullopt_t, const optional<T>& x) noexcept
-{
-  return (!x);
-}
-
-template <class T> constexpr bool operator!=(const optional<T>& x, nullopt_t) noexcept
-{
-  return bool(x);
-}
-
-template <class T> constexpr bool operator!=(nullopt_t, const optional<T>& x) noexcept
-{
-  return bool(x);
-}
-
-template <class T> constexpr bool operator<(const optional<T>&, nullopt_t) noexcept
-{
-  return false;
-}
-
-template <class T> constexpr bool operator<(nullopt_t, const optional<T>& x) noexcept
-{
-  return bool(x);
-}
-
-template <class T> constexpr bool operator<=(const optional<T>& x, nullopt_t) noexcept
-{
-  return (!x);
-}
-
-template <class T> constexpr bool operator<=(nullopt_t, const optional<T>&) noexcept
-{
-  return true;
-}
-
-template <class T> constexpr bool operator>(const optional<T>& x, nullopt_t) noexcept
-{
-  return bool(x);
-}
-
-template <class T> constexpr bool operator>(nullopt_t, const optional<T>&) noexcept
-{
-  return false;
-}
-
-template <class T> constexpr bool operator>=(const optional<T>&, nullopt_t) noexcept
-{
-  return true;
-}
-
-template <class T> constexpr bool operator>=(nullopt_t, const optional<T>& x) noexcept
-{
-  return (!x);
-}
-
-
-
-// 20.5.10, Comparison with T
-template <class T> constexpr bool operator==(const optional<T>& x, const T& v)
-{
-  return bool(x) ? *x == v : false;
-}
-
-template <class T> constexpr bool operator==(const T& v, const optional<T>& x)
-{
-  return bool(x) ? v == *x : false;
-}
-
-template <class T> constexpr bool operator!=(const optional<T>& x, const T& v)
-{
-  return bool(x) ? *x != v : true;
-}
-
-template <class T> constexpr bool operator!=(const T& v, const optional<T>& x)
-{
-  return bool(x) ? v != *x : true;
-}
-
-template <class T> constexpr bool operator<(const optional<T>& x, const T& v)
-{
-  return bool(x) ? *x < v : true;
-}
-
-template <class T> constexpr bool operator>(const T& v, const optional<T>& x)
-{
-  return bool(x) ? v > *x : true;
-}
-
-template <class T> constexpr bool operator>(const optional<T>& x, const T& v)
-{
-  return bool(x) ? *x > v : false;
-}
-
-template <class T> constexpr bool operator<(const T& v, const optional<T>& x)
-{
-  return bool(x) ? v < *x : false;
-}
-
-template <class T> constexpr bool operator>=(const optional<T>& x, const T& v)
-{
-  return bool(x) ? *x >= v : false;
-}
-
-template <class T> constexpr bool operator<=(const T& v, const optional<T>& x)
-{
-  return bool(x) ? v <= *x : false;
-}
-
-template <class T> constexpr bool operator<=(const optional<T>& x, const T& v)
-{
-  return bool(x) ? *x <= v : true;
-}
-
-template <class T> constexpr bool operator>=(const T& v, const optional<T>& x)
-{
-  return bool(x) ? v >= *x : true;
-}
-
-
-// Comparison of optional<T&> with T
-template <class T> constexpr bool operator==(const optional<T&>& x, const T& v)
-{
-  return bool(x) ? *x == v : false;
-}
-
-template <class T> constexpr bool operator==(const T& v, const optional<T&>& x)
-{
-  return bool(x) ? v == *x : false;
-}
-
-template <class T> constexpr bool operator!=(const optional<T&>& x, const T& v)
-{
-  return bool(x) ? *x != v : true;
-}
-
-template <class T> constexpr bool operator!=(const T& v, const optional<T&>& x)
-{
-  return bool(x) ? v != *x : true;
-}
-
-template <class T> constexpr bool operator<(const optional<T&>& x, const T& v)
-{
-  return bool(x) ? *x < v : true;
-}
-
-template <class T> constexpr bool operator>(const T& v, const optional<T&>& x)
-{
-  return bool(x) ? v > *x : true;
-}
-
-template <class T> constexpr bool operator>(const optional<T&>& x, const T& v)
-{
-  return bool(x) ? *x > v : false;
-}
-
-template <class T> constexpr bool operator<(const T& v, const optional<T&>& x)
-{
-  return bool(x) ? v < *x : false;
-}
-
-template <class T> constexpr bool operator>=(const optional<T&>& x, const T& v)
-{
-  return bool(x) ? *x >= v : false;
-}
-
-template <class T> constexpr bool operator<=(const T& v, const optional<T&>& x)
-{
-  return bool(x) ? v <= *x : false;
-}
-
-template <class T> constexpr bool operator<=(const optional<T&>& x, const T& v)
-{
-  return bool(x) ? *x <= v : true;
-}
-
-template <class T> constexpr bool operator>=(const T& v, const optional<T&>& x)
-{
-  return bool(x) ? v >= *x : true;
-}
-
-// Comparison of optional<T const&> with T
-template <class T> constexpr bool operator==(const optional<const T&>& x, const T& v)
-{
-  return bool(x) ? *x == v : false;
-}
-
-template <class T> constexpr bool operator==(const T& v, const optional<const T&>& x)
-{
-  return bool(x) ? v == *x : false;
-}
-
-template <class T> constexpr bool operator!=(const optional<const T&>& x, const T& v)
-{
-  return bool(x) ? *x != v : true;
-}
-
-template <class T> constexpr bool operator!=(const T& v, const optional<const T&>& x)
-{
-  return bool(x) ? v != *x : true;
-}
-
-template <class T> constexpr bool operator<(const optional<const T&>& x, const T& v)
-{
-  return bool(x) ? *x < v : true;
-}
-
-template <class T> constexpr bool operator>(const T& v, const optional<const T&>& x)
-{
-  return bool(x) ? v > *x : true;
-}
-
-template <class T> constexpr bool operator>(const optional<const T&>& x, const T& v)
-{
-  return bool(x) ? *x > v : false;
-}
-
-template <class T> constexpr bool operator<(const T& v, const optional<const T&>& x)
-{
-  return bool(x) ? v < *x : false;
-}
-
-template <class T> constexpr bool operator>=(const optional<const T&>& x, const T& v)
-{
-  return bool(x) ? *x >= v : false;
-}
-
-template <class T> constexpr bool operator<=(const T& v, const optional<const T&>& x)
-{
-  return bool(x) ? v <= *x : false;
-}
-
-template <class T> constexpr bool operator<=(const optional<const T&>& x, const T& v)
-{
-  return bool(x) ? *x <= v : true;
-}
-
-template <class T> constexpr bool operator>=(const T& v, const optional<const T&>& x)
-{
-  return bool(x) ? v >= *x : true;
-}
-
-
-// 20.5.12, Specialized algorithms
-template <class T>
-void swap(optional<T>& x, optional<T>& y) noexcept(noexcept(x.swap(y)))
-{
-  x.swap(y);
-}
-
-
-template <class T>
-constexpr optional<typename decay<T>::type> make_optional(T&& v)
-{
-  return optional<typename decay<T>::type>(constexpr_forward<T>(v));
-}
-
-template <class X>
-constexpr optional<X&> make_optional(reference_wrapper<X> v)
-{
-  return optional<X&>(v.get());
-}
-
-
-} // namespace experimental
-} // namespace std
-
-namespace std
-{
-  template <typename T>
-  struct hash<std::experimental::optional<T>>
-  {
-    typedef typename hash<T>::result_type result_type;
-    typedef std::experimental::optional<T> argument_type;
-
-    constexpr result_type operator()(argument_type const& arg) const {
-      return arg ? std::hash<T>{}(*arg) : result_type{};
-    }
-  };
-
-  template <typename T>
-  struct hash<std::experimental::optional<T&>>
-  {
-    typedef typename hash<T>::result_type result_type;
-    typedef std::experimental::optional<T&> argument_type;
-
-    constexpr result_type operator()(argument_type const& arg) const {
-      return arg ? std::hash<T>{}(*arg) : result_type{};
-    }
-  };
-}
-
-# undef TR2_OPTIONAL_REQUIRES
-# undef TR2_OPTIONAL_ASSERTED_EXPRESSION
-
-# endif //___OPTIONAL_HPP___
diff --git a/src/3rdparty/optional/ottd_optional.h b/src/3rdparty/optional/ottd_optional.h
deleted file mode 100644
--- a/src/3rdparty/optional/ottd_optional.h
+++ /dev/null
@@ -1,33 +0,0 @@
-/*
- * This file is part of OpenTTD.
- * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
- * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
- * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
- */
-
-/** @file ottd_optional.h Header to select between native. */
-
-#ifndef OTTD_OPTIONAL_H
-#define OTTD_OPTIONAL_H
-
-#if defined(__has_include)
-#	if __has_include(<version>)
-#		include <version>
-#	endif
-#endif
-
-#if (__cplusplus >= 201703L) || (defined(__cpp_lib_optional) && __cpp_lib_optional >= 201606L)
-
-/* Native std::optional. */
-#include <optional>
-namespace opt = std;
-
-#else
-
-/* No std::optional, use local copy instead. */
-#include "optional.hpp"
-namespace opt = std::experimental;
-
-#endif
-
-#endif /* OTTD_OPTIONAL_H */
diff --git a/src/ini_type.h b/src/ini_type.h
--- a/src/ini_type.h
+++ b/src/ini_type.h
@@ -12,7 +12,7 @@
 
 #include "fileio_type.h"
 #include <string>
-#include "3rdparty/optional/ottd_optional.h"
+#include <optional>
 
 /** Types of groups */
 enum IniGroupType {
@@ -25,7 +25,7 @@ enum IniGroupType {
 struct IniItem {
 	IniItem *next;                    ///< The next item in this group
 	std::string name;                 ///< The name of this item
-	opt::optional<std::string> value; ///< The value of this item
+	std::optional<std::string> value; ///< The value of this item
 	std::string comment;              ///< The comment associated with this item
 
 	IniItem(struct IniGroup *parent, const std::string &name);