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4513. meta::exception::what() should be consteval

Section: 21.4.4 [meta.reflection.exception] Status: New Submitter: Jonathan Wakely Opened: 2026-01-28 Last modified: 2026-02-13

Priority: 2

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Discussion:

As mentioned in LWG 4494(i), meta::exception::what() can be (and should be) marked consteval now that CWG 3117 has been approved.

[2026-02-13; Reflector poll.]

Set priority to 2 after reflector poll.

Discussion about whether this is really necessary, or redundant.

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;

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

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

    constexpr consteval const char* what() const noexcept override;
    consteval u8string_view u8what() const noexcept;
    consteval info from() const noexcept;
    consteval source_location where() const noexcept;
  };
}

    …

    constexpr consteval const char* what() const noexcept override;

    -5- Constant when: what_.has_value() is true.