monostate is effectively an empty After the Boost formal review, the implementation has been
+changed to provide the strong exception safety guarantee,
+instead of basic. expected has been removed.
This variant implementation has two distinguishing features:
It’s never "valueless", that is, variant<T1, T2, …, Tn> has an
+invariant that it always contains a valid value of one of the types
+T1, T2, …, Tn.
It provides the strong exception safety guarantee on assignment and
+emplace.
This is achieved with the use of double storage, unless all of the +contained types have a non-throwing move constructor.
+…
+…
+The main differences between this implementation and std::variant are:
No valueless-by-exception state: valueless_by_exception() always
+returns false.
Strong exception safety guarantee on assignment and emplace.
emplace first constructs the new value and then destroys the old one;
+in the single storage case, this translates to constructing a temporary
+and then moving it into place.
A converting constructor from, e.g. variant<int, float> to
+variant<float, double, int> is provided as an extension.
The reverse operation, going from variant<float, double, int> to
+variant<int, float> is provided as the member function subset<U…>.
+(This operation can throw if the current state of the variant cannot be
+represented.)
variant<T…> is not (yet) trivial when all contained types are trivial,
+as mandated by C++17.
The C++20 additions and changes to std::variant have not yet been
+implemented.
This library is API compatible with std::variant. As such, its interface
+is different from Boost.Variant’s. For example, visitation is performed via
+visit instead of apply_visitor.
Recursive variants are not supported.
+Double storage is used instead of temporary heap backup. This variant is
+always "stack-based", it never allocates, and never throws bad_alloc on
+its own.