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4500. constant_wrapper wording problems

Section: 21.3.5 [const.wrap.class] Status: New Submitter: Matthias Wippich Opened: 2026-01-07 Last modified: 2026-01-08

Priority: Not Prioritized

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Discussion:

During resolution of LWG 4383⁽ⁱ⁾ prefix and postfix increment and decrement operators were changed to

template<constexpr-param T>
  constexpr auto operator++(this T) noexcept -> constant_wrapper<++Y>
    { return {}; }
template<constexpr-param T>
  constexpr auto operator++(this T, int) noexcept ->
    constant_wrapper<Y++> { return {}; }
template<constexpr-param T>
  constexpr auto operator--(this T) noexcept -> constant_wrapper<--Y>
    { return {}; }
template<constexpr-param T>
  constexpr auto operator--(this T, int) noexcept ->
    constant_wrapper<Y--> { return {}; }

However, we do not actually specify what Y is. Additionally, the assignment operator has been changed to

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

This is grammatically not valid C++. The assignment must be parenthesized.

Proposed resolution:

This wording is relative to N5032.

1. Modify 21.3.5 [const.wrap.class], class template constant_wrapper synopsis, as indicated:

   struct cw-operators { // exposition only
     …
     // pseudo-mutators
     template<constexpr-param T>
       constexpr auto operator++(this T) noexcept
         -> constant_wrapper<++Y (++T::value)> { return {}; }
     template<constexpr-param T>
       constexpr auto operator++(this T, int) noexcept
         -> constant_wrapper<Y++ (T::value++)> { return {}; }
     template<constexpr-param T>
       constexpr auto operator--(this T) noexcept
         -> constant_wrapper<--Y (--T::value)> { return {}; }
     template<constexpr-param T>
       constexpr auto operator--(this T, int) noexcept
         -> constant_wrapper<Y-- (T::value--)> { return {}; }
     …
   };
   
   template<cw-fixed-value X, class>
   struct constant_wrapper : cw-operators {
     static constexpr const auto & value = X.data;
     using type = constant_wrapper;
     using value_type = decltype(X)::type;
   
     template<constexpr-param R>
       constexpr auto operator=(R) const noexcept
         -> constant_wrapper<(X = R::value)> { return {}; }
     constexpr operator decltype(auto)() const noexcept { return value; }
   };