9a632fd684
When using `std::variant` with non-trivial types, we need to go through multiple bases to find the `_Which` member. The MSVC STL implements this in `xsmf_control.h` which conditionally adds/deletes copy/move constructors/operators. We now go to `_Variant_base` (the holder of `_Which`). This inherits from `_Variant_storage`, which is our entry point to finding the n-th storage (going through `_Tail`).