Struct inheritance

Decided

D-0032, settled by the language’s author: part of spec §18. Older text cites it as 18.32.

Single inheritance, written struct Button : UiElement. The base comes first in memory, so &Derived → &Base and ^Derived → ^Base are implicit and free, while a by-value Derived where a Base is wanted is an error — C++’s slicing, which drops what the derived type adds. A base’s fields and methods are the derived type’s, unqualified inside its methods, and neither may be redeclared. A base with a constructor is constructed through it, Button(…) : UiElement(r), and a type with a constructor is not built with a struct literal. Dispatch is static unless written: virtual fn opts one method in, override fn replaces one, both checked in both directions, and a body-less virtual fn makes the struct abstract — no value of it exists, only a & or a ^ to one. Base::method(self) calls the base’s version without dispatch, and drop becomes virtual as soon as anything in the chain is. A fourth visibility, protected, opens a member to the types that inherit it (§4). Implementations are inherited and a more specific impl replaces the inherited one, statically. This reverses §7.3’s old “there is no inheritance”, which was never a decision of the language’s author (§7.3, §4)