`T` where a `T?` is wanted

Decided

D-0053, settled by the language’s author: part of spec §18. Older text cites it as 18.53.

Converts, as at return. A T becomes a T? wherever a T? is expected — a let with that type, an argument, an assignment — and where if or switch arms are joined and some arm is already a T? or none. Never the other way: a T? to a T still takes or. Chosen over return-only, which made state-machine switches write every arm out as an optional (§9.1)