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4468. §[const.wrap.class] "operator decltype(auto)" is ill-formed

Section: 21.3.5 [const.wrap.class] Status: New Submitter: Jiang An Opened: 2025-11-07 Last modified: 2025-11-10

Priority: Not Prioritized

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Discussion:

Following the approval of CWG 1670 in Kona 2025, the following declaration in class template constant_wrapper, 21.3.5 [const.wrap.class], is ill-formed:

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

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.3.5 [const.wrap.class], class template constant_wrappersynopsis, 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 = typename decltype(X)::type;
   
     […]
     constexpr operator decltype(autovalue)() const noexcept { return value; }
   };