A union variant that is an existing struct

Decided

D-0045, settled by the language’s author: part of spec §18. Older text cites it as 18.45.

A bare variant names a struct in scope and carries it; a variant with no payload is written Point {}. union ButtonState { DefaultState TransitionState } holds a DefaultState or a TransitionState, each built as that struct is built — its fields, defaults and visibility — and bound by case DefaultState(d) as that struct, methods included. A bare name that is no struct in scope, names something else, or names a generic struct is an error (DZ0551) rather than a variant with nothing in it: giving the bare spelling both meanings would let a misspelling, or a struct added or removed elsewhere, silently change what a union holds. Point {} is already how a variant with an empty field list was written, so nothing new is spelled; Shape::Point, with no braces, still names one (§7.5)