Omnibus fresh object subtype reduction / contextual type issue

[RyanCavanaugh]

// Problem 1: Error spans are useless in contextually-typed array/object literal cases
type Kind = 'foo' | 'ball';
// Error is on declared identifier instead of bad element
const ka: Kind[] = [
    'foo', 'ball', 'xxxx', 'foo', 'ball'
];
// Error is on declared identifier instead of bad key
const ko: { [s: string]: Kind } = {
    'x': 'foo',
    'y': 'xxxx',
    'z': 'foo'
};

// Problem 2: Subtype reduction screwage in the presence of contextual type
type OK = { ok?: boolean };
// No error??
const o1: OK = '??' ? { ok: 'wat' } : { };

// Problem 3: Aliased subtype reduction screwage
const obj = '??' ? { ok: 'wat' } : {};
// No error??
const o2: OK = obj;

Hypotheses:

• Computing a union type in the presence of a contextual type is a waste of time
  • If we do this, we can produce good error spans?
• Unioning { x: T } with a fresh { } should produce { x?: T }
• Unioning a fresh object literal with { } should do something special
• Unioning fresh { x: number, y: number } | { x: number } should produce { x: number, y?: number}

[zpdDG4gta8XKpMCd]

pardon me, can we keep { x: T } | {} as it is: { x: T } | {} ? or what is the way to encode an empty (no properties) object?

[ahejlsberg]

@Aleksey-Bykov The issue is that { x: T } | {} is too weak. Anything is assignable to it because anything is assignable to {}. The idea is to instead infer the type { x: T? } which says that if there is an x property it must be of type T.

[zpdDG4gta8XKpMCd]

it's sad that there is no place for a true empty object {} which doesn't accept any properties

we use it as yet another unit type in addition to null and undefined and []

[zpdDG4gta8XKpMCd]

so the problem here is the syntax {} is overloaded with semantics:

• {} should mean an object with no properties as it literally stands
• {} should mean a top type assignable from anything
  we need a way to distinguish the two Exact Types #12936