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4523. constant_wrapper should assign to value

Section: 21.3.5 [const.wrap.class] Status: New Submitter: Tomasz Kamiński Opened: 2026-02-17 Last modified: 2026-02-17

Priority: Not Prioritized

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Discussion:

During resolution of LWG 4383(i) assignment operator of constant_wrapper, was changed to: 

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

This is incorrect (and inconsistent with other operators), as we assign to X that is object of specialization of exposition only cw-fixed-value, instead of value.

Proposed resolution:

This wording is relative to N5032 with changes from 4500(i).

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

    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<(valueX = R::value)> { return {}; }

  constexpr operator decltype(auto)() const noexcept { return value; }
};