Using a `T?` when both outcomes continue

Decided

D-0036, settled by the language’s author: part of spec §18. Older text cites it as 18.36.

if let name = value { … } else { … }. The binding holds what or would produce — the T of a T?, the &mut T of a ^T — in the first block only. Chosen over narrowing m after if m != none, which needs flow-sensitive types and works only on locals, and over “or only”, which leaves “both branches, then carry on” with no spelling. It does not take a fallible call, whose else would drop the Error unseen (§9.5); binding the error in else is left for later. m == none / m != none test presence; two optionals do not compare (§9.4). How the binding holds the value is revised by 18.61: if let p = ptr is now a copy, and the &mut T is if let &mut p = ptr