A link to make type-safe unions:

Eggs.Variant - Part II (the constexpr experience)

by K-ballo

From the article:

Ruminations on the development of Eggs.Variant, a C++11/14 generic, type-safe, discriminated union. Part I explored a straightforward implementation based on untyped raw storage appropriate to hold any of the variant members. It was noted, however, that such an implementation would never be constexpr-aware. It's time to throw it away and start from scratch in order to properly support constexpr...