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Omnibus fresh object subtype reduction / contextual type issue #19236
Description
// 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}
Activity
zpdDG4gta8XKpMCd commentedon Oct 17, 2017
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 commentedon Oct 17, 2017
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 anxproperty it must be of typeT.zpdDG4gta8XKpMCd commentedon Oct 17, 2017
it's sad that there is no place for a true empty object
{}which doesn't accept any propertieswe use it as yet another unit type in addition to null and undefined and []
zpdDG4gta8XKpMCd commentedon Oct 17, 2017
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 anythingwe need a way to distinguish the two Exact Types #12936