Multiple ChoiceHard 45s
variant visit return type & exhaustive
`std::visit` requires that the visitor be callable for:
Options
- 1Only the currently-active alternative at runtime, checked with a hidden switch before dispatch.
- 2The common base type shared by all alternatives in the variant, to allow uniform handling.
- 3Only the first alternative of the variant by default, with the rest reachable via overloads.
- 4Every alternative in the variant (exhaustively), otherwise it is a compile error.
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