Improved type inference for object literals

[ahejlsberg]

This PR improves type inference for multiple object literals occurring in the same context. When multiple object literal types contribute to a union type, we now normalize the object literal types such that all properties are present in each constituent of the union type. For example:

const obj = test ? { text: "hello" } : {};  // { text: string } | { text?: undefined }
const s = obj.text;  // string | undefined

Previously we inferred type {} for obj and the second line subsequently caused an error because obj would appear to have no properties. That obviously wasn't ideal.

The key insight behind this PR is that for object literal types we know that unspecified properties always have the value undefined. This is unlike regular object types where unspecified properties might have any value. Based on this fact we implement the following new rules:

• When checking a type relationship between a source type S and a target type T, if T is an object literal type, any excess properties in S are required to have type undefined (normally such excess properties are permitted to have any type).
• When widening a union type, we normalize all constituent types originating in object literals by adding optional properties of type undefined to each constituent as appropriate such that all constituents have the same set of property names.
• When inferring multiple object literal types for a type parameter, the object literal types are normalized into a single inferred union type.

An example of object literal normalization:

// let obj: { a: number, b: number } |
//     { a: string, b?: undefined } |
//     { a?: undefined, b?: undefined }
let obj = [{ a: 1, b: 2 }, { a: "abc" }, {}][0];
obj.a;  // string | number | undefined
obj.b;  // number | undefined

Nested object literals are likewise normalized:

// let obj: { kind: string, pos: { x: number, y: number, a?: undefined, b?: undefined } } |
//     { kind: string, pos: { a: string, x?: undefined, y?: undefined, b?: undefined } | { b: number, x?: undefined, y?: undefined, a?: undefined } }
let obj = [{ kind: 'a', pos: { x: 0, y: 0 } }, { kind: 'b', pos: test ? { a: "x" } : { b: 0 } }][0];
obj.kind;   // string
obj.pos;    // { x: number, y: number, a?: undefined, b?: undefined } | { a: string, x?: undefined, y?: undefined, b?: undefined } | { b: number, x?: undefined, y?: undefined, a?: undefined }
obj.pos.x;  // number | undefined
obj.pos.y;  // number | undefined
obj.pos.a;  // string | undefined
obj.pos.b;  // number | undefined

Multiple object literal type inferences for the same type parameter are collapsed into a single normalized union type:

declare function f<T>(...items: T[]): T;
// let obj: { a: number, b: number } |
//     { a: string, b?: undefined } |
//     { a?: undefined, b?: undefined }
let obj = f({ a: 1, b: 2 }, { a: "abc" }, {});
obj.a;  // string | number | undefined
obj.b;  // number | undefined

Fixes #19236.

[mhegazy]

files #19613 to track that.

[RafaelSalguero]

Does this PR also fixes #19843?