build: MSVC workaround for empty base classes

[Radnyx]

MSVC is knocking out bugs to get mp-units building. One of those bugs occurs here in unit.h:

template<typename... Us, Unit U>
[[nodiscard]] consteval Unit auto get_common_unit_in(common_unit<Us...>, U u)
{
  // ...
  constexpr auto canonical_u = get_canonical_unit(u);
  // ...
}

The call to get_canonical_unit tries to copy u, which is outside the evaluation context of canonical_u. Normally this would be an error, but since Unit types have no members to copy (they are "empty"), this is actually permitted by the C++ standard.

We get this wrong due to an age-old layout problem. The structs scaled_unit_impl and derived_unit_impl inherit from empty base classes, so they should be considered empty as well. But due to historical reasons, we don't grant this if there are multiple base classes.

This is fixable on the compiler side. In the meantime, adding __declspec(empty_bases) is the workaround to get standard behavior (MSVC docs, introduced in Visual Studio 2015).

[mpusz]

Thank you! I wasn't aware of it and had to fix many similar lines in mp-units before to make MSVC happier. I was changing u to U{} and similar everywhere. Units are not the only ones affected. Similar issues may happen with dimension and quantity_spec classes, as they have the same design and usage. If you didn't approach problems so far, it means that I've probably fixed most of those already 😉

If only I had known about this workaround, it would have been much easier to get the MSVC compliance earlier.

[mpusz]

BTW, I really appreciate the MSVC developers' efforts here! 💜