DT regression from Improved type inference for object literals

[mhegazy]

#19513 broke a DefinitelyTyped test. (Tested by building locally both with this commit and with the previous commit 4895d16) Go to DefinitelyTyped/types/baconjs and run tsc to see the error.

EDIT: Same for d3-selection-multi.

It's hard to say what exactly changed. Something that used to be assignable is no longer assignable.
In baconjs the error is:

baconjs-tests.ts(187,28): error TS2345: Argument of type '(groupedStream: EventStream<string, { id: number; type: string; val?: number; }>) => EventStream<...' is not assignable to parameter of type '(groupedStream: EventStream<{}, { id: number; type: string; val: number; } | { id: number; type: ...'.
  Types of parameters 'groupedStream' and 'groupedStream' are incompatible.
    Type 'EventStream<{}, { id: number; type: string; val: number; } | { id: number; type: string; val?: un...' is not assignable to type 'EventStream<string, { id: number; type: string; val?: number; }>'.
      Type '{}' is not assignable to type 'string'.
baconjs-tests.ts(188,22): error TS7006: Parameter 'groupedStream' implicitly has an 'any' type.
baconjs-tests.ts(188,62): error TS7006: Parameter 'acc' implicitly has an 'any' type.
baconjs-tests.ts(188,67): error TS7006: Parameter 'x' implicitly has an 'any' type.
baconjs-tests.ts(189,22): error TS7006: Parameter 'sum' implicitly has an 'any' type.

In d3-selection-multi the error is:

d3-selection-multi-tests.ts(37,29): error TS2345: Argument of type '(this: HTMLAnchorElement, d: string, i: number, g: ArrayLike<HTMLAnchorElement> | HTMLAnchorEleme...' is not assignable to parameter of type 'ValueFn<HTMLAnchorElement, string, ValueMap<HTMLAnchorElement, string>>'.
  Type '{ id?: undefined; } | { id: string; }' is not assignable to type 'ValueMap<HTMLAnchorElement, string>'.
    Type '{ id?: undefined; }' is not assignable to type 'ValueMap<HTMLAnchorElement, string>'.
      Property 'id' is incompatible with index signature.
        Type 'undefined' is not assignable to type 'string | number | boolean | ValueFn<HTMLAnchorElement, string, string | number | boolean | null> ...'.
d3-selection-multi-tests.ts(92,34): error TS2345: Argument of type '(this: HTMLAnchorElement, d: string, i: number, g: ArrayLike<HTMLAnchorElement> | HTMLAnchorEleme...' is not assignable to parameter of type 'ValueFn<HTMLAnchorElement, string, ValueMap<HTMLAnchorElement, string>>'.
  Type '{ href?: undefined; } | { href: string; }' is not assignable to type 'ValueMap<HTMLAnchorElement, string>'.
    Type '{ href?: undefined; }' is not assignable to type 'ValueMap<HTMLAnchorElement, string>'.
      Property 'href' is incompatible with index signature.
        Type 'undefined' is not assignable to type 'string | number | boolean | ValueFn<HTMLAnchorElement, string, string | number | boolean | null> ...'.
d3-selection-multi-tests.ts(121,31): error TS2345: Argument of type '(this: HTMLAnchorElement, d: string, i: number, g: ArrayLike<HTMLAnchorElement> | HTMLAnchorEleme...' is not assignable to parameter of type 'ValueFn<HTMLAnchorElement, string, ValueMap<HTMLAnchorElement, string>>'.
  Type '{ id?: undefined; } | { id: string; }' is not assignable to type 'ValueMap<HTMLAnchorElement, string>'.
    Type '{ id?: undefined; }' is not assignable to type 'ValueMap<HTMLAnchorElement, string>'.
      Property 'id' is incompatible with index signature.
        Type 'undefined' is not assignable to type 'string | number | boolean | ValueFn<HTMLAnchorElement, string, string | number | boolean | null> ...'.

[mhegazy]

 var events = [
                {id: 1, type: "add", val: 3},
                {id: 2, type: "add", val: -1},
                {id: 1, type: "add", val: 2},
                {id: 2, type: "cancel"},
                {id: 3, type: "add", val: 2},
                {id: 3, type: "cancel"},
                {id: 1, type: "add", val: 1},
                {id: 1, type: "add", val: 2},
                {id: 1, type: "cancel"}
            ];

events used to be inferred to { id: number; type: string; } | { id: number; type: string; val: number} now it is { id: number; type: string; val: number} | { id: number; type: string; val?: undefined}. which is not assignable from { id: number; type: string; val?: number} any more

seems like the new type is worse..

[ahejlsberg]

The new type is fine and in fact more correct. The real issue is that we don't consider { ... , p: T | U } assignable to { ... , p: T } | { ... , p: U }. We've gotten this request several times before, it may be time to get it done.

BTW, it's interesting to observe that an object type with one or more properties that have union types should behave equivalently to a union of object types with all possible permutations of those property types. This of course expands exponentially, so real life scenarios typically only have one property for which the expansion takes place, and it may be that those are the only ones we really care about.

Anders Hejlsberg (@ahejlsberg) The error in d3-selection-multi looks different -- we are inferring a type for a callback and then issuing an error that the inferred type isn't assignable to the declared callback type.

[weswigham]

BTW, it's interesting to observe that an object type with one or more properties that have union types should behave equivalently to a union of object types with all possible permutations of those property types. This of course expands exponentially, so real life scenarios typically only have one property for which the expansion takes place, and it may be that those are the only ones we really care about.

#14865 Is open to track this already.

[mhegazy]

closing in favor of #14865

[ahejlsberg]

The three new errors in d3-selection-multi-tests.ts are all correct. We now infer a return type of { id: string } | { id?: undefined } where previously we just inferred {}. The more precise return type then causes an error because undefined isn't a permitted property value (probably an oversight since null appears to be permitted).