`move` on a copyable value

Decided

D-0033, settled by the language’s author: part of spec §18. Older text cites it as 18.33.

It moves. The source is moved-from, a later use is an error, and it is reset to its defaults at runtime — the same as for any type. The compiler had been treating it as a copy, which §6.4 never said; one rule for every type is simpler to learn, and it keeps a generic body meaning one thing for every argument. To keep using a copyable value, leave out move (§6.4)