namespace std {
  namespace experimental::inline fundamentals_v3 {

    // 6.1.2.1, Overview
    template <class T> class propagate_const;

    // 6.1.2.9, Relational operators
    template <class T>
      constexpr bool operator==(const propagate_const<T>& pt, nullptr_t);
    template <class T>
      constexpr bool operator==(nullptr_t, const propagate_const<T>& pu);

    template <class T>
      constexpr bool operator!=(const propagate_const<T>& pt, nullptr_t);
    template <class T>
      constexpr bool operator!=(nullptr_t, const propagate_const<T>& pu);

    template <class T, class U>
      constexpr bool operator==(const propagate_const<T>& pt, const propagate_const<U>& pu);
    template <class T, class U>
      constexpr bool operator!=(const propagate_const<T>& pt, const propagate_const<U>& pu);
    template <class T, class U>
      constexpr bool operator<(const propagate_const<T>& pt, const propagate_const<U>& pu);
    template <class T, class U>
      constexpr bool operator>(const propagate_const<T>& pt, const propagate_const<U>& pu);
    template <class T, class U>
      constexpr bool operator<=(const propagate_const<T>& pt, const propagate_const<U>& pu);
    template <class T, class U>
      constexpr bool operator>=(const propagate_const<T>& pt, const propagate_const<U>& pu);

    template <class T, class U>
      constexpr bool operator==(const propagate_const<T>& pt, const U& u);
    template <class T, class U>
      constexpr bool operator!=(const propagate_const<T>& pt, const U& u);
    template <class T, class U>
      constexpr bool operator<(const propagate_const<T>& pt, const U& u);
    template <class T, class U>
      constexpr bool operator>(const propagate_const<T>& pt, const U& u);
    template <class T, class U>
      constexpr bool operator<=(const propagate_const<T>& pt, const U& u);
    template <class T, class U>
      constexpr bool operator>=(const propagate_const<T>& pt, const U& u);

    template <class T, class U>
      constexpr bool operator==(const T& t, const propagate_const<U>& pu);
    template <class T, class U>
      constexpr bool operator!=(const T& t, const propagate_const<U>& pu);
    template <class T, class U>
      constexpr bool operator<(const T& t, const propagate_const<U>& pu);
    template <class T, class U>
      constexpr bool operator>(const T& t, const propagate_const<U>& pu);
    template <class T, class U>
      constexpr bool operator<=(const T& t, const propagate_const<U>& pu);
    template <class T, class U>
      constexpr bool operator>=(const T& t, const propagate_const<U>& pu);

    // 6.1.2.10, Specialized algorithms
    template <class T>
      constexpr void swap(propagate_const<T>& pt, propagate_const<T>& pt2) noexcept(see below);

    // 6.1.2.11, Underlying pointer access
    template <class T>
      constexpr const T& get_underlying(const propagate_const<T>& pt) noexcept;
    template <class T>
      constexpr T& get_underlying(propagate_const<T>& pt) noexcept;

  } // namespace experimental::inline fundamentals_v3

  // 6.1.2.12, Hash support
  template <class T> struct hash;
  template <class T>
    struct hash<experimental::fundamentals_v3::propagate_const<T>>;

  // 6.1.2.13, Comparison function objects
  template <class T> struct equal_to;
  template <class T>
    struct equal_to<experimental::fundamentals_v3::propagate_const<T>>;
  template <class T> struct not_equal_to;
  template <class T>
    struct not_equal_to<experimental::fundamentals_v3::propagate_const<T>>;
  template <class T> struct less;
  template <class T>
    struct less<experimental::fundamentals_v3::propagate_const<T>>;
  template <class T> struct greater;
  template <class T>
    struct greater<experimental::fundamentals_v3::propagate_const<T>>;
  template <class T> struct less_equal;
  template <class T>
    struct less_equal<experimental::fundamentals_v3::propagate_const<T>>;
  template <class T> struct greater_equal;
  template <class T>
    struct greater_equal<experimental::fundamentals_v3::propagate_const<T>>;

} // namespace std

namespace std::experimental::inline fundamentals_v3 {

  // 6.2.2, Class templates scope_exit, scope_fail, and scope_success
  template <class EF>
    class scope_exit;
  template <class EF>
    class scope_fail;
  template <class EF>
    class scope_success;

  // 6.2.3, Class template unique_resource
  template <class R, class D>
    class unique_resource;

  // 6.2.3.6, unique_resource creation
  template <class R, class D, class S=decay_t<R>>
    unique_resource<decay_t<R>, decay_t<D>>
      make_unique_resource_checked(R&& r, const S& invalid, D&& d) noexcept(see below);

} // namespace std::experimental::inline fundamentals_v3

The class templates scope_exit, scope_fail, and scope_success define scope guards that wrap a function object to be called on their destruction.

In this subclause, the placeholder scope-guard denotes each of these class templates. In descriptions of the class members, scope-guard refers to the enclosing class.

namespace std::experimental::inline fundamentals_v3 {

  template <class EF> class scope-guard {
  public:
    template <class EFP>
      explicit scope-guard(EFP&& f) noexcept(see below);
    scope-guard(scope-guard&& rhs) noexcept(see below);

    scope-guard(const scope-guard&) = delete;
    scope-guard& operator=(const scope-guard&) = delete;
    scope-guard& operator=(scope-guard&&) = delete;

    ~scope-guard () noexcept(see below);

    void release() noexcept;

  private:
    EF exit_function;                                 // exposition only
    bool execute_on_destruction{true};                // exposition only
    int uncaught_on_creation{uncaught_exceptions()};  // exposition only
  };

  template <class EF>
    scope-guard(EF) -> scope-guard<EF>;

}  // namespace std::experimental::inline fundamentals_v3

The class template scope_exit is a general-purpose scope guard that calls its exit function when a scope is exited. The class templates scope_fail and scope_success share the scope_exit interface, only the situation when the exit function is called differs.

void grow(vector<int>& v) {
  scope_success guard([]{ cout << "Good!" << endl; });
  v.resize(1024);
}

[ Note: If the exit function object of a scope_success or scope_exit object refers to a local variable of the function where it is defined, e.g., as a lambda capturing the variable by reference, and that variable is used as a return operand in that function, it is possible for that variable to already have been returned when the scope-guard’s destructor executes, calling the exit function. This can lead to surprising behavior. — end note ]

Template argument EF shall be a function object type (C++20 §20.14), lvalue reference to function, or lvalue reference to function object type. If EF is an object type, it shall meet the Cpp17Destructible requirements (C++20 Table 30). Given an lvalue g of type remove_reference_t<EF>, the expression g() shall be well-formed.

The constructor parameter f in the following constructors shall be a reference to a function or a reference to a function object (C++20 §20.14).

template <class EFP>
explicit scope-guard(EFP&& f) noexcept(
    is_nothrow_constructible_v<EF, EFP> ||
    is_nothrow_constructible_v<EF, EFP&>);

scope-guard(scope-guard&& rhs) noexcept(see below)

~scope_exit() noexcept(true);

~scope_fail() noexcept(true);

~scope_success() noexcept(noexcept(exit_function()));

void release() noexcept;

#include <type_traits>

namespace std::experimental::inline fundamentals_v3 {

  // 6.3.2, Other type transformations
  template <class> class invocation_type; // not defined
  template <class F, class... ArgTypes> class invocation_type<F(ArgTypes...)>;
  template <class> class raw_invocation_type; // not defined
  template <class F, class... ArgTypes> class raw_invocation_type<F(ArgTypes...)>;

  template <class T>
    using invocation_type_t = typename invocation_type<T>::type;
  template <class T>
    using raw_invocation_type_t = typename raw_invocation_type<T>::type;

  // 6.3.3, Detection idiom
  struct nonesuch;

  template <template<class...> class Op, class... Args>
    using is_detected = see below;
  template <template<class...> class Op, class... Args>
    inline constexpr bool is_detected_v
      = is_detected<Op, Args...>::value;
  template <template<class...> class Op, class... Args>
    using detected_t = see below;
  template <class Default, template<class...> class Op, class... Args>
    using detected_or = see below;
  template <class Default, template<class...> class Op, class... Args>
    using detected_or_t = typename detected_or<Default, Op, Args...>::type;
  template <class Expected, template<class...> class Op, class... Args>
    using is_detected_exact = is_same<Expected, detected_t<Op, Args...>>;
  template <class Expected, template<class...> class Op, class... Args>
    inline constexpr bool is_detected_exact_v
      = is_detected_exact<Expected, Op, Args...>::value;
  template <class To, template<class...> class Op, class... Args>
    using is_detected_convertible = is_convertible<detected_t<Op, Args...>, To>;
  template <class To, template<class...> class Op, class... Args>
    inline constexpr bool is_detected_convertible_v
      = is_detected_convertible<To, Op, Args...>::value;

} // namespace std::experimental::inline fundamentals_v3

This subclause contains templates that may be used to transform one type to another following some predefined rule.

Each of the templates in this subclause shall be a TransformationTrait (C++20 §20.15.1).

Within this section, define the invocation parameters of INVOKE(f, t1, t2, ..., tN) as follows, in which T1 is the possibly cv-qualified type of t1 and U1 denotes T1& if t1 is an lvalue or T1&& if t1 is an rvalue:

• When f is a pointer to a member function of a class T the invocation parameters are U1 followed by the parameters of f matched by t2, ..., tN.
• When N == 1 and f is a pointer to member data of a class T the invocation parameter is U1.
• If f is a class object, the invocation parameters are the parameters matching t1, ..., tN of the best viable function (C++20 §12.4.3) for the arguments t1, ..., tN among the function call operators and surrogate call functions of f.
• In all other cases, the invocation parameters are the parameters of f matching t1, ... tN.

In all of the above cases, if an argument tI matches the ellipsis in the function's parameter-declaration-clause, the corresponding invocation parameter is defined to be the result of applying the default argument promotions (C++20 §7.6.1.2) to tI.

struct S {
  int f(double const &) const;
  void operator()(int, int);
  void operator()(char const *, int i = 2, int j = 3);
  void operator()(...);
};

Access checking is performed as if in a context unrelated to Fn and ArgTypes. Only the validity of the immediate context of the expression is considered. [ Note: The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note ]

The member raw_invocation_type<Fn(ArgTypes...)>::type shall be defined as follows. If the expression INVOKE(declval<Fn>(), declval<ArgTypes>()...) is ill-formed when treated as an unevaluated operand (C++20 §7), there shall be no member type. Otherwise:

• Let R denote result_of_t<Fn(ArgTypes...)>.
• Let the types Ti be the invocation parameters of INVOKE(declval<Fn>(), declval<ArgTypes>()...).
• Then the member type shall name the function type R(T1, T2, ...).

The member invocation_type<Fn(ArgTypes...)>::type shall be defined as follows. If raw_invocation_type<Fn(ArgTypes...)>::type does not exist, there shall be no member type. Otherwise:

• Let A1, A2, … denote ArgTypes...
• Let R(T1, T2, …) denote raw_invocation_type_t<Fn(ArgTypes...)>
• Then the member type shall name the function type R(U1, U2, …) where Ui is decay_t<Ai> if declval<Ai>() is an rvalue otherwise Ti.

struct nonesuch {
  ~nonesuch() = delete;
  nonesuch(nonesuch const&) = delete;
  void operator=(nonesuch const&) = delete;
};

nonesuch has no default constructor (C++20 §11.4.4) or initializer-list constructor (C++20 §9.4.4), and is not an aggregate (C++20 §9.4.1).

template <class Default, class AlwaysVoid,
          template<class...> class Op, class... Args>
struct DETECTOR { // exposition only
  using value_t = false_type;
  using type = Default;
};

template <class Default, template<class...> class Op, class... Args>
struct DETECTOR<Default, void_t<Op<Args...>>, Op, Args...> { // exposition only
  using value_t = true_type;
  using type = Op<Args...>;
};

template <template<class...> class Op, class... Args>
  using is_detected = typename DETECTOR<nonesuch, void, Op, Args...>::value_t;

template <template<class...> class Op, class... Args>
  using detected_t = typename DETECTOR<nonesuch, void, Op, Args...>::type;

template <class Default, template<class...> class Op, class... Args>
  using detected_or = DETECTOR<Default, void, Op, Args...>;

// archetypal helper alias for a copy assignment operation:
template <class T>
  using copy_assign_t = decltype(declval<T&>() = declval<T const &>());

// plausible implementation for the is_assignable type trait:
template <class T>
  using is_copy_assignable = is_detected<copy_assign_t, T>;

// plausible implementation for an augmented is_assignable type trait
// that also checks the return type:
template <class T>
  using is_canonical_copy_assignable = is_detected_exact<T&, copy_assign_t, T>;

// archetypal helper alias for a particular type member:
template <class T>
  using diff_t = typename T::difference_type;

// alias the type member, if it exists, otherwise alias ptrdiff_t:
template <class Ptr>
  using difference_type = detected_or_t<ptrdiff_t, diff_t, Ptr>;

The specification of all declarations within subclause 7.2 are the same as the corresponding declarations, as specified in C++20 §20.14.16.2, unless explicitly specified otherwise. [ Note: std::experimental::function uses std::bad_function_call, there is no additional type std::experimental::bad_function_call — end note ] .

namespace std {
  namespace experimental::inline fundamentals_v3 {

    template<class> class function; // undefined

    template<class R, class... ArgTypes>
    class function<R(ArgTypes...)> {
    public:
      using result_type = R;

      using allocator_type = std::pmr::polymorphic_allocator<>;

      function() noexcept;
      function(nullptr_t) noexcept;
      function(const function&);
      function(function&&);
      template<class F> function(F);
      function(allocator_arg_t, const allocator_type&) noexcept;
      function(allocator_arg_t, const allocator_type&, nullptr_t) noexcept;
      function(allocator_arg_t, const allocator_type&, const function&);
      function(allocator_arg_t, const allocator_type&, function&&);
      template<class F> function(allocator_arg_t, const allocator_type&, F);

      function& operator=(const function&);
      function& operator=(function&&);
      function& operator=(nullptr_t) noexcept;
      template<class F> function& operator=(F&&);
      template<class F> function& operator=(reference_wrapper<F>);

      ~function();

      void swap(function&);

      explicit operator bool() const noexcept;

      R operator()(ArgTypes...) const;

      const type_info& target_type() const noexcept;
      template<class T> T* target() noexcept;
      template<class T> const T* target() const noexcept;

      allocator_type get_allocator() const noexcept;
    };

  } // namespace experimental::inline fundamentals_v3
} // namespace std

A function object stores an allocator object of type std::pmr::polymorphic_allocator<>, which it uses to allocate memory for its internal data structures. In the function constructors, the allocator is initialized (before the target object, if any) as follows:

• For the move constructor, the allocator is initialized from f.get_allocator(), where f is the parameter of the constructor.
• For constructors having a first parameter of type allocator_arg_t, the allocator is initialized from the second parameter.
• For all other constructors, the allocator is value-initialized.

In all cases, the allocator of a parameter having type function&& is unchanged. If the constructor creates a target object, that target object is initialized by uses-allocator construction with the allocator and other target object constructor arguments. [ Note: If a constructor parameter of type experimental::function&& has an allocator equal to that of the object being constructed, the implementation can often transfer ownership of the target rather than constructing a new one. — end note ]

function& operator=(const function& f);

function& operator=(function&& f);

function& operator=(nullptr_t) noexcept;

template<class F> function& operator=(F&& f);

template<class F> function& operator=(reference_wrapper<F> f) noexcept;

namespace std::experimental::inline fundamentals_v3 {

  template <class W> class observer_ptr {
    using pointer = add_pointer_t<W>;            // exposition-only
    using reference = add_lvalue_reference_t<W>; // exposition-only
  public:
    // publish our template parameter and variations thereof
    using element_type = W;

    // 8.2.2, observer_ptr constructors
    // default constructor
    constexpr observer_ptr() noexcept;

    // pointer-accepting constructors
    constexpr observer_ptr(nullptr_t) noexcept;
    constexpr explicit observer_ptr(pointer) noexcept;

    // copying constructors (in addition to the implicit copy constructor)
    template <class W2> constexpr observer_ptr(observer_ptr<W2>) noexcept;

    // 8.2.3, observer_ptr observers
    constexpr pointer get() const noexcept;
    constexpr reference operator*() const;
    constexpr pointer operator->() const noexcept;
    constexpr explicit operator bool() const noexcept;

    // 8.2.4, observer_ptr conversions
    constexpr explicit operator pointer() const noexcept;

    // 8.2.5, observer_ptr modifiers
    constexpr pointer release() noexcept;
    constexpr void reset(pointer = nullptr) noexcept;
    constexpr void swap(observer_ptr&) noexcept;
  }; // observer_ptr<>

} // namespace std::experimental::inline fundamentals_v3

A non-owning pointer, known as an observer, is an object o that stores a pointer to a second object, w. In this context, w is known as a watched object. [ Note: There is no watched object when the stored pointer is nullptr. — end note ] An observer takes no responsibility or ownership of any kind for its watched object, if any; in particular, there is no inherent relationship between the lifetimes of o and w.

Specializations of observer_ptr shall meet the requirements of a Cpp17CopyConstructible and Cpp17CopyAssignable type. The template parameter W of an observer_ptr shall not be a reference type, but may be an incomplete type.

[ Note: The uses of observer_ptr include clarity of interface specification in new code, and interoperability with pointer-based legacy code. — end note ]

An instance of resource_adaptor<Allocator> is an adaptor that wraps a memory_resource interface around Allocator. In order that resource_adaptor<X<T>> and resource_adaptor<X<U>> are the same type for any allocator template X and types T and U, resource_adaptor<Allocator> is rendered as an alias to a class template such that Allocator is rebound to a char value type in every specialization of the class template. The requirements on this class template are defined below. The name resource_adaptor_imp is for exposition only and is not normative, but the definitions of the members of that class, whatever its name, are normative. In addition to the Cpp17Allocator requirements (C++20 §16.5.3.5), the parameter to resource_adaptor shall meet the following additional requirements:

• typename allocator_traits<Allocator>::pointer shall be identical to typename allocator_traits<Allocator>::value_type*.
• typename allocator_traits<Allocator>::const_pointer shall be identical to typename allocator_traits<Allocator>::value_type const*.
• typename allocator_traits<Allocator>::void_pointer shall be identical to void*.
• typename allocator_traits<Allocator>::const_void_pointer shall be identical to void const*.

// The name resource_adaptor_imp is for exposition only.
template <class Allocator>
class resource_adaptor_imp : public memory_resource {
  // for exposition only
  Allocator m_alloc;

public:
  using allocator_type = Allocator;

  resource_adaptor_imp() = default;
  resource_adaptor_imp(const resource_adaptor_imp&) = default;
  resource_adaptor_imp(resource_adaptor_imp&&) = default;

  explicit resource_adaptor_imp(const Allocator& a2);
  explicit resource_adaptor_imp(Allocator&& a2);

  resource_adaptor_imp& operator=(const resource_adaptor_imp&) = default;

  allocator_type get_allocator() const { return m_alloc; }

protected:
  virtual void* do_allocate(size_t bytes, size_t alignment);
  virtual void do_deallocate(void* p, size_t bytes, size_t alignment);

  virtual bool do_is_equal(const memory_resource& other) const noexcept;
};

template <class Allocator>
  using resource_adaptor = typename resource_adaptor_imp<
    typename allocator_traits<Allocator>::template rebind_alloc<char>>;

ostream_joiner writes (using operator<<) successive elements onto the output stream from which it was constructed. The delimiter that it was constructed with is written to the stream between every two Ts that are written. It is not possible to get a value out of the output iterator. Its only use is as an output iterator in situations like

while (first != last)
  *result++ = *first++;

ostream_joiner is defined as

namespace std::experimental::inline fundamentals_v3 {

  template <class DelimT, class charT = char, class traits = char_traits<charT> >
  class ostream_joiner {
  public:
    using char_type = charT;
    using traits_type = traits;
    using ostream_type = basic_ostream<charT, traits>;
    using iterator_category = output_iterator_tag;
    using value_type = void;
    using difference_type = void;
    using pointer = void;
    using reference = void;

    ostream_joiner(ostream_type& s, const DelimT& delimiter);
    ostream_joiner(ostream_type& s, DelimT&& delimiter);
    template<typename T>
    ostream_joiner& operator=(const T& value);
    ostream_joiner& operator*() noexcept;
    ostream_joiner& operator++() noexcept;
    ostream_joiner& operator++(int) noexcept;

  private:
    ostream_type* out_stream; // exposition only
    DelimT delim;             // exposition only
    bool first_element;       // exposition only
  };

} // namespace std::experimental::inline fundamentals_v3

#include <random>

namespace std::experimental::inline fundamentals_v3 {

  // 11.1.2, Function template randint
  template <class IntType>
  IntType randint(IntType a, IntType b);
  void reseed();
  void reseed(default_random_engine::result_type value);

} // namespace std::experimental::inline fundamentals_v3

A separate per-thread engine of type default_random_engine (C++20 §26.6.5), initialized to an unpredictable state, shall be maintained for each thread.

template<class IntType>
IntType randint(IntType a, IntType b);

void reseed();void reseed(default_random_engine::result_type value);