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4494. Add consteval to std::meta::exception defaulted member functions

Section: 21.4.4 [meta.reflection.exception] Status: New Submitter: Marek Polacek Opened: 2025-12-16 Last modified: 2025-12-20

Priority: Not Prioritized

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Discussion:

CWG 3115 states that every function of consteval-only type shall be an immediate function. This caused a problem for std::meta::exception::what() which was marked constexpr but not consteval. To be able to mark it consteval, we had to tweak the rules about overriding by a consteval virtual function (CWG 3117 ).

But 21.4.4 [meta.reflection.exception] still defines std::meta::exception such that it contains these defaulted special member functions:

exception(const exception&) = default;
exception(exception&&) = default;

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

which aren't consteval (and since they're not templates, they won't be promoted to consteval as per P2564). I propose to make the four functions consteval:

consteval exception(const exception&) = default;
consteval exception(exception&&) = default;

consteval exception& operator=(const exception&) = default;
consteval exception& operator=(exception&&) = default;

Proposed resolution:

This wording is relative to N5032.

1. Modify 21.4.4 [meta.reflection.exception] as indicated:

   namespace std::meta {
     class exception : public std::exception {
     private:
       optional<string> what_;  // exposition only
       u8string u8what_;        // exposition only
       info from_;              // exposition only
       source_location where_;  // exposition only
     public:
       consteval exception(u8string_view what, info from,
                           source_location where = source_location::current()) noexcept;
                           
       consteval exception(string_view what, info from,
                           source_location where = source_location::current()) noexcept;
                           
       consteval exception(const exception&) = default;
       consteval exception(exception&&) = default;
       
       consteval exception& operator=(const exception&) = default;
       consteval exception& operator=(exception&&) = default;
       
       constexpr const char* what() const noexcept override;
       consteval u8string_view u8what() const noexcept;
       consteval info from() const noexcept;
       consteval source_location where() const noexcept;
     };
   }