On using constexpr and unions, with insights into the design of the currently-proposed std::optional<T>:

Constexpr Unions

by Andrzej Krzemieński

I assume you are already familiar with constexpr functions. (If not, see a short introduction here.) [Ed.: We linked to that article recently, so you've seen it if you've been following isocpp.org.]

In this post I wanted to share my experience with using unions in constant expressions. Unions are not very popular due to type-safety hole they open, but they offer some capabilities that I found priceless when working with Fernando Cacciola on std::optional proposal.

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