[Question]: array of elements of union type

[matt77hias]

Have you considered using an underlying array containing elements of union type for the static_vector implementation?

Fwiiw I implemented my own StaticVector< T, N, A > with the following goals:

• usable in constexpr expressions (unlike EASTL's eastl::fixed_vector);
• not relying on operator new/dynamic memory allocation to support compile-time initialization and runtime usage (unlike C++20's std::vector);
• not requiring value types to be default constructible and/or trivially destructible (unlike wrapping a std::array/C-style array).

I was inspired by std::optional< T > to achieve the latter.

One cannot use an array of std::optional< T > elements directly, because besides wrapping a union, std::optional< T > needs to keep track of the active union member as well. This bookkeeping is redundant for each array element. Furthermore, std::optional< T > has a larger size than the value it wraps, which does not allow for a data() member method.

Instead my StaticVector< T, N, A > holds an array of unions, and uses the size() member method to keep track of all active union members in the array using the following class invariants:

• The elements at [0, size()) have the value_type;
• The elements at [size(), capacity()) have a trivial dummy type.

A slightly stripped down version can be found at https://developercommunity.visualstudio.com/t/failure-was-caused-by-attempting-to-acce/1515298 (MSVC has some issue(s) unlike clang and gcc).