Constraints:
No recursive composition: Vector and Swizzle are not the same class.
Function argument template deduction: Vector and swizzles have base classes that are of the same template (partial specialization).
Standard layout: No common class in the vector and swizzle inheritance hierarchies.
Shared operators: Vector and swizzle share a template partial specialization.
constexpr: Vector and swizzle shared template partial specialization takes CRTP.
Slicing: Vector and swizzle shared template partial specialization is not easily expressed (to avoid accidentally taking it as an argument and slicing)
Final layout: