AK: Variant the union thing [2/N] - #26916
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This makes the overload resolution logic a lot cleaner. If we have needs for duplicate types we can rely on `DedupAndApply` to strip duplicate types before they reach Variant proper.
For constructing the Storage we use a sentinel value (`VariantIndex`) to pass along the desired depth/type into the constructor chain. This is done as we cant easily start the lifetime of the alternatives after the union was instantiated. Upcoming c++26 features should fix this (see trivial unions (P3074R7) and std::start_lifetime (P3726R2)) For move and copy assignment we destroy the active member, and then replace the whole storage, trivial unions may also make this nicer. Also Overload resolution now returns the chosen overload wrapper, which now also holds the index of that overload, which avoids re-walking the type list to get the index.
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This does two things:
This allows stripping some extra logic from the overload resolution and makes the next step easier
On clang this might also be a bit nicer on the compiler