A union variant that owns something

Decided

D-0052, settled by the language’s author: part of spec §18. Older text cites it as 18.52.

§7.5’s proposal adopted: a union owns its active payload. It is dropped with the union, dropped before another variant is assigned, and moved by moving the active payload. A union is copyable only when every variant is. This is also what lets a variant carry a struct with a constructor, copy, drop, base or virtual method (§7.5)