std::constant_wrapper

Document #: P2781R4
Date: 2023-06-10
Project: Programming Language C++
Audience: LEWG-I
LEWG
Reply-to: Matthias Kretz
<>
Zach Laine
<>

1 Changelog

1.1 Changes since R0

1.2 Changes since R1

1.3 Changes since R2

1.4 Changes since R3

2 Relationship to previous work

This paper is co-authored by the authors of P2725R1 (“std::integral_constant Literals”) and P2772R0 (“std::integral_constant literals do not suffice — constexpr_t?”). This paper supersedes both of those previous papers.

3 The ergonomics of std::integral_constant<int> are bad

std::integral_constant<int> is used in lots of places to communicate a constant integral value to a given interface. The length of its spelling makes it very verbose. Fortunately, we can do a lot better.

Before
After 
// From P2630R1
auto const sir =
  std::strided_index_range{std::integral_constant<size_t, 0>{},
                           std::integral_constant<size_t, 10>{},
                           3};
auto y = submdspan(x, sir);
 
auto y = submdspan(x, std::strided_index_range{
    std::cw<0>, std::cw<10>, 3});

The “after” case above would require that std::strided_index_range be changed; that is not being proposed here. The point of the example is to show the relative convenience of std::integral_constant versus the proposed std::constant_wrapper.

3.1 Replacing the uses of std::integral_constant is not enough

Parameters passed to a constexpr function lose their constexpr-ness when used inside the function. Replacing std::integral_constant with std::constant_wrapper has the potential to improve a lot more uses of compile-time constants than just integrals; what about all the other constexpr-friendly C++ types?

Consider:

template<typename T>
struct my_complex
{
    T re, im;
};

inline constexpr short foo = 2;

template<typename T>
struct X
{
    void f(auto c)
    {
        // c is to be used as a constexpr value here
    }
};

We would like to be able to call X::f() with a value, and have that value keep its constexpr-ness. Let’s introduce a template “constant_wrapper” that holds a constexpr value that it is given as an non-type template parameter.

namespace std {
  template<auto X>
  struct constant_wrapper
  {
    using value_type = remove_cvref_t<decltype(X)>;
    using type = constant_wrapper;

    constexpr operator value_type() const { return X; }
    static constexpr value_type value = X;

    // The rest of the members are discussed below ....
  };
}

Now we can write this.

template<typename T>
void g(X<T> x)
{
    x.f(std::constant_wrapper<1>{});
    x.f(std::constant_wrapper<2uz>{});
    x.f(std::constant_wrapper<3.0>{});
    x.f(std::constant_wrapper<4.f>{});
    x.f(std::constant_wrapper<foo>{});
    x.f(std::constant_wrapper<my_complex(1.f, 1.f)>{});
}

Let’s now add a constexpr variable template with a shorter name, say cw.

namespace std {
  template<auto X>
  inline constexpr constant_wrapper<X> cw{};
}

And now we can write this.

template<typename T>
void g(X<T> x)
{
    x.f(std::cw<1>);
    x.f(std::cw<2uz>);
    x.f(std::cw<3.0>);
    x.f(std::cw<4.f>);
    x.f(std::cw<foo>);
    x.f(std::cw<my_complex(1.f, 1.f)>);
}

3.2 The difference in template parameters to std::constant_wrapper and std::cw

If you look at the wording below, you will see that std::cw takes an auto NTTP, whereas std::constant_wrapper takes an auto NTTP X, and an exposition-only parameter adl-type which is defaulted to remove_cvref_t<decltype(X)>. Why is this? As the adl-type name implies, ADL! Even though the type of X is deduced with or without adl-type, without it some natural uses of constant_wrapper cease to work. For instance:

auto f = std::cw<strlit("foo")>; // Using the strlit from later in this paper.
std::cout << f << "\n";

The stream insertion breaks without the adl-type parameter. adl-type is strlit</*...*/>, which pulls strlit’s operator<< into consideration during ADL. Note that this ADL support is imperfect. The use op operator<< above is due to the way the operator overload is declared:

friend std::ostream & operator<<(std::ostream & os, strlit l) { /* ...*/ }

If it is instead declared as a non-friend:

template<size_t N>
std::ostream & operator<<(std::ostream & os, strlit<N> l) { /* ...*/ }

… ADL’s help doesn’t suffice. The deduction of N is not possible from a type that isn’t a strlit<N> itself (e.g. base class) even if it is implicitly convertible to strlit<N>.

4 Making constant_wrapper more useful

constant_wrapper is essentially a wrapper. It takes a value X of some structural type value_type, and represents X in such a way that we can continue to use X as a compile-time constant, regardless of context. As such, constant_wrapper should be implicitly convertible to value_type; this is already reflected in the design presented above. For the same reason, constant_wrapper should provide all the operations that the underlying type has. Though we cannot predict what named members the underlying type value_type has, we can guess at all the operator overloads it might have.

So, by adding conditionally-defined overloads for all the overloadable operators, we can make constant_wrapper as natural to use as many of the types it might wrap.

namespace std {
  template<auto X>
  struct constant_wrapper {
    using value_type = remove_cvref_t<decltype(X)>;
    using type = constant_wrapper;

    constexpr operator value_type() const { return X; }
    static constexpr value_type value = X;

    // unary -
    template<auto Y = X>
      constexpr constant_wrapper<-Y> operator-() const { return {}; }

    // binary + and -
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<U::value + V::value> operator+(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<U::value - V::value> operator-(U, V) { return {}; }

    // etc... (full listing later)
  };
}

These operators are defined in such a way that they behave just like the operations on underlying the U and V values would, including promotions and coercions. For example:

static_assert(std::is_same_v<
              decltype(std::cw<42> - std::cw<13u>),
              std::constant_wrapper<29u>>);

Each operation is only defined if the underlying operation on X is defined. Each operation additionally requires that the result of the underlying operation have a structural type.

All the overloadable operations are included, even the index and call operators. The rationale for this is that a user may want to make some sort of compile-time domain-specific embedded language using operator overloading, and having all but a couple of the operators specified would frustrate that effort. The only exception to this is operator-> which must eventually return a pointer type, which is not very useful at compile time.

The only downside to adding std::constant_wrapper::operator() is that it would represent a break from the design of std::integral_constant, making it an imperfect drop-in replacement for that template. Nullary std::constant_wrapper::operator() with the same semantics as std::integral_constant::operator() is defined when requires (!std::invocable<value_type>) is true, so this incompatibility is truly a corner case.

The operators are designed to interoperate with other types and templates that have a constexpr static value member. This works with std::constant_wrappers of course, but also std::integral_constants, and user-provided types as well. For example:

struct my_type { constexpr static int value = 42; };

void foo()
{
    constexpr auto zero = my_type{} - std::cw<42>;  // Ok.
    // ...
}

Note that the addition of these operators is in line with the poll:

“Add a new robust integral constant type with all the numerical operators, as proposed in P2772R0, and use that for these literals instead of std::integral_constant”?

SF
F
N
A
SA
4 7 1 1 1

… taken in the 2023-01-17 Library Evolution telecon.

Note that the one SA said he would not be opposed if the word “integral” was stricken from the poll, and the design of std::constant_wrapper is not limited to integral types.

5 What about strings?

As pointed out on the reflector, std::cw<"foo"> does not work, because of language rules. However, it’s pretty easy for users to add an NTTP-friendly string wrapper type, and then use that with std::cw<>.

template<size_t N>
struct strlit
{
    constexpr strlit(char const (&str)[N]) { std::copy_n(str, N, value); }

    template<size_t M>
    constexpr bool operator==(strlit<M> rhs) const
    {
        return std::ranges::equal(bytes_, rhs.bytes_);
    }

    friend std::ostream & operator<<(std::ostream & os, strlit l)
    {
        assert(!l.value[N - 1] && "value must be null-terminated");
        return os.write(l.value, N - 1);
    }

    char value[N];
};

int main()
{
    auto f = std::cw<strlit("foo")>;
    std::cout << f; // Prints "foo".
}

6 An example using operator()

The addition of non-arithmetic operators may seem academic at first. However, consider this constexpr-friendly parser combinator mini-library.

namespace parse {

    template<typename L, typename R>
    struct or_parser;

    template<size_t N>
    struct str_parser
    {
        template<size_t M>
        constexpr bool operator()(strlit<M> lit) const
        {
            return lit == str_;
        }
        template<typename P>
        constexpr auto operator|(P parser) const
        {
            return or_parser<str_parser, P>{*this, parser};
        }
        strlit<N> str_;
    };

    template<typename L, typename R>
    struct or_parser
    {
        template<size_t M>
        constexpr bool operator()(strlit<M> lit) const
        {
            return l_(lit) || r_(lit);
        }
        template<typename P>
        constexpr auto operator|(P parser) const
        {
            return or_parser<or_parser, P>{*this, parser};
        }
        L l_;
        R r_;
    };

}

int foo()
{
    constexpr parse::str_parser p1{strlit("neg")};
    constexpr parse::str_parser p2{strlit("incr")};
    constexpr parse::str_parser p3{strlit("decr")};

    constexpr auto p = p1 | p2 | p3;

    constexpr bool matches_empty = p(strlit(""));
    static_assert(!matches_empty);
    constexpr bool matches_pos = p(strlit("pos"));
    static_assert(!matches_pos);
    constexpr bool matches_decr = p(strlit("decr"));
    static_assert(matches_decr);
}

(This relies on the strlit struct shown just previously.)

Say we wanted to use the templates in namespace parser along side other values, like ints and floats. We would want that not to break our std::constant_wrapper expressions. Having to work around the absence of std::constant_wrapper::operator() would require us to write a lot more code. Here is the equivalent of the function foo() above, but with all the variables wrapped using std::cw.

int bar()
{
    constexpr parse::str_parser p1{strlit("neg")};
    constexpr parse::str_parser p2{strlit("incr")};
    constexpr parse::str_parser p3{strlit("decr")};

    constexpr auto p_ = std::cw<p1> | std::cw<p2> | std::cw<p3>;

    constexpr bool matches_empty_ = p_(std::cw<strlit("")>);
    static_assert(!matches_empty_);
    constexpr bool matches_pos_ = p_(std::cw<strlit("pos")>);
    static_assert(!matches_pos_);
    constexpr bool matches_decr_ = p_(std::cw<strlit("decr")>);
    static_assert(matches_decr_);
}

As you can see, everything works as it did before. The presence of operator() does not enable any new functionality, it just keeps code that happens to use it from breaking.

7 What about the mutating operators?

It may seem at first that these operators are nonsensical, since all the operations on a constant_wrapper must be nonmutating.

However, some DSLs may wish to use these operations with atypical semantics.

struct weirdo
{
    constexpr int operator++() const { return 1; }
};
auto result = ++std::cw<weirdo{}>;

result is obviously std::cw<1> here, and no mutation occurred. You can imagine a more elaborate use case, say a library that is used to create expression templates. For example:

auto expr = std::cw<var0> += std::cw<var1>;

In this case, var0 and var1 would be some terminal types in the expression template library, and operator+= would return a constexpr expression tree, rather than mutating the left side of the +=.

These operators are now part of the proposal, based on this LEWG poll from Kona 2023:

“We should add mutating operations (i.e. #define IF_LEWG_SAYS_SO 1 and ++ and --) to P2781R3”

SF
F
N
A
SA
2 6 5 2 0

8 What about operator->?

We’re not proposing it, because of its very specific semantics – it must yield a pointer, or something that eventually does. That’s not a very useful operation during constant evaluation.

9 Convertibility to and from std::integral_constant

During the LEWG reviews, some attendees suggested that inter-conversions between std::integral_constant and std::constant_wrapper would be useful. The important thing to remember is that we want deduction to occur when calling functions that take a std::constant_wrapper, including the std::constant_wrapper operator overloads. Conversions and deductions are at odds with one another, because deducing parameter types disables the conversion rules.

If you look at the operator overloads proposed here, you will see that they are deduction operations at their most essential. The types of the parameters do not matter, except that each conveys a value that is a core constant expression because it is embedded in the type system. The fact that a std::constant_wrapper conveys that value instead of a std::integral_constant is immaterial, and in fact the operators are written in such a way that they operate on either template (as long as at least one parameter is a specialization of std::constant_wrapper). Users can and should write their code using these kinds of values-as-types in a similar way. Relying on conversions is a less-useful way to get interoperability.

10 Design

10.1 Add constant_wrapper

namespace std {
  template<auto X,
           class adl-type = remove_cvref_t<decltype(X)>>     // exposition only
    struct constant_wrapper;

  template <class T>
    concept constexpr-param =                                // exposition only
      requires { typename constant_wrapper<T::value>; };
  template <class T>
    concept derived-from-constexpr =                         // exposition only
      derived_from<T, constant_wrapper<T::value>>;
  template <class T, class SelfT>
    concept lhs-constexpr-param =                            // exposition only
      constexpr-param<T> && (derived_from<T, SelfT> || !derived-from-constexpr<T>);

  template<auto X, class adl-type>
  struct constant_wrapper {
    using value_type = adl-type;
    using type = constant_wrapper;

    constexpr operator value_type() const { return X; }
    static constexpr value_type value = X;

    template <constexpr-param U>
      constexpr constant_wrapper<(X = U::value)> operator=(U) const { return {}; }

    template<auto Y = X>
      constexpr constant_wrapper<+Y> operator+() const { return {}; }
    template<auto Y = X>
      constexpr constant_wrapper<-Y> operator-() const { return {}; }
    template<auto Y = X>
      constexpr constant_wrapper<~Y> operator~() const { return {}; }
    template<auto Y = X>
      constexpr constant_wrapper<!Y> operator!() const { return {}; }
    template<auto Y = X>
      constexpr constant_wrapper<&Y> operator&() const { return {}; }
    template<auto Y = X>
      constexpr constant_wrapper<*Y> operator*() const { return {}; }

    template<auto Y = X, class... Args>
      constexpr constant_wrapper<Y(Args::value...)> operator()(Args... args) const { return {}; }
    template<auto Y = X, class... Args>
      constexpr constant_wrapper<Y[Args::value...]> operator[](Args... args) const { return {}; }
    constexpr value_type operator()() const requires (!std::invocable<value_type>) { return X; }

    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<U::value + V::value> operator+(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<U::value - V::value> operator-(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<U::value * V::value> operator*(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<U::value / V::value> operator/(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<U::value % V::value> operator%(U, V) { return {}; }

    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value << V::value)> operator<<(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value >> V::value)> operator>>(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<U::value & V::value> operator&(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<U::value | V::value> operator|(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<U::value ^ V::value> operator^(U, V) { return {}; }

    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<U::value && V::value> operator&&(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<U::value || V::value> operator||(U, V) { return {}; }

    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value <=> V::value)> operator<=>(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value == V::value)> operator==(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value != V::value)> operator!=(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value < V::value)> operator<(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value > V::value)> operator>(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value <= V::value)> operator<=(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value >= V::value)> operator>=(U, V) { return {}; }

    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value, V::value)> operator,(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value ->* V::value)> operator->*(U, V) { return {}; }

    template <auto Y = X>
      constexpr constant_wrapper<++Y> operator++() { return {}; }
    template <auto Y = X>
      constexpr constant_wrapper<Y++> operator++(int) { return {}; }
    template <auto Y = X>
      constexpr constant_wrapper<--Y> operator--() { return {}; }
    template <auto Y = X>
      constexpr constant_wrapper<Y--> operator--(int) { return {}; }

    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value += V::value)> operator+=(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value -= V::value)> operator-=(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value *= V::value)> operator*=(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value /= V::value)> operator/=(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value %= V::value)> operator%=(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value &= V::value)> operator&=(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value |= V::value)> operator|=(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value ^= V::value)> operator^=(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value <<= V::value)> operator<<=(U, V) { return {}; }
    template <lhs-constexpr-param<type> U, constexpr-param V>
      friend constexpr constant_wrapper<(U::value >>= V::value)> operator>>=(U, V) { return {}; }
  };

  template<auto X>
    inline constexpr constant_wrapper<X> cw{};
}

10.2 Add a feature macro

Add a new feature macro, __cpp_lib_constant_wrapper.

11 Implementation experience

Look up a few lines to see an implementation of std::constant_wrapper. At the time of this writing, there is one caveat: operator[]() looks correct to the authors, but does not work in any compiler tested, due to the very limited multi-variate operator[] support in even the latest compilers.

Additionally, an integral_constant with most of the operator overloads has been a part of Boost.Hana since its initial release in May of 2016. Its operations have been used by many, many users.

12 Wording

Add the following to [meta.type.synop], after false_type:

template<auto X,
         class adl-type = remove_cvref_t<decltype(X)>>     // exposition only
  struct constant_wrapper;

template <class T>
  concept constexpr-param =                                // exposition only
    requires { typename constant_wrapper<T::value>; };
template <class T>
  concept derived-from-constexpr =                         // exposition only
    derived_from<T, constant_wrapper<T::value>>;
template <class T, class SelfT>
  concept lhs-constexpr-param =                            // exposition only
    constexpr-param<T> && (derived_from<T, SelfT> || !derived-from-constexpr<T>);

template<auto X>
  inline constexpr constant_wrapper<X> cw;

Add the following to [meta.help], after integral_constant:

template<auto X, class adl-type>
struct constant_wrapper {
  using value_type = adl-type;
  using type = constant_wrapper;

  constexpr operator value_type() const { return X; }
  static constexpr value_type value = X;

  template <constexpr-param U>
    constexpr constant_wrapper<(X = U::value)> operator=(U) const { return {}; }

  template<auto Y = X>
    constexpr constant_wrapper<+Y> operator+() const { return {}; }
  template<auto Y = X>
    constexpr constant_wrapper<-Y> operator-() const { return {}; }
  template<auto Y = X>
    constexpr constant_wrapper<~Y> operator~() const { return {}; }
  template<auto Y = X>
    constexpr constant_wrapper<!Y> operator!() const { return {}; }
  template<auto Y = X>
    constexpr constant_wrapper<&Y> operator&() const { return {}; }
  template<auto Y = X>
    constexpr constant_wrapper<*Y> operator*() const { return {}; }

  template<auto Y = X, class... Args>
    constexpr constant_wrapper<Y(Args::value...)> operator()(Args... args) const { return {}; }
  template<auto Y = X, class... Args>
    constexpr constant_wrapper<Y[Args::value...]> operator[](Args... args) const { return {}; }
  constexpr value_type operator()() const requires (!std::invocable<value_type>) { return X; }

  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<U::value + V::value> operator+(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<U::value - V::value> operator-(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<U::value * V::value> operator*(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<U::value / V::value> operator/(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<U::value % V::value> operator%(U, V) { return {}; }

  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value << V::value)> operator<<(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value >> V::value)> operator>>(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<U::value & V::value> operator&(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<U::value | V::value> operator|(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<U::value ^ V::value> operator^(U, V) { return {}; }

  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<U::value && V::value> operator&&(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<U::value || V::value> operator||(U, V) { return {}; }

  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value <=> V::value)> operator<=>(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value == V::value)> operator==(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value != V::value)> operator!=(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value < V::value)> operator<(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value > V::value)> operator>(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value <= V::value)> operator<=(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value >= V::value)> operator>=(U, V) { return {}; }

  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value, V::value)> operator,(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value ->* V::value)> operator->*(U, V) { return {}; }

  template <auto Y = X>
    constexpr constant_wrapper<++Y> operator++() { return {}; }
  template <auto Y = X>
    constexpr constant_wrapper<Y++> operator++(int) { return {}; }
  template <auto Y = X>
    constexpr constant_wrapper<--Y> operator--() { return {}; }
  template <auto Y = X>
    constexpr constant_wrapper<Y--> operator--(int) { return {}; }

  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value += V::value)> operator+=(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value -= V::value)> operator-=(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value *= V::value)> operator*=(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value /= V::value)> operator/=(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value %= V::value)> operator%=(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value &= V::value)> operator&=(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value |= V::value)> operator|=(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value ^= V::value)> operator^=(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value <<= V::value)> operator<<=(U, V) { return {}; }
  template <lhs-constexpr-param<type> U, constexpr-param V>
    friend constexpr constant_wrapper<(U::value >>= V::value)> operator>>=(U, V) { return {}; }
};

template<auto X>
  inline constexpr constant_wrapper<X> cw{};

2 The class template constant_wrapper aids in metaprogramming by ensuring that the evaluation of expressions comprised entirely of constant_wrappers are core constant expressions ([expr.const]), regardless of the context in which they appear. In particular, this enables use of constant_wrapper values that are passed as arguments to constexpr functions to be used as template parameters.

3 The variable template cw is provided as a convenient way to nominate constant_wrapper values.

Add to [version.syn]:

#define __cpp_lib_constant_wrapper XXXXXXL // also in <type_traits>