Type union not matched starting from TS 5.1 about typescript

[ahejlsberg]

Yeah, there's an issue here. This is a simplified repro:

type Foo = { kind: "a" | "b", value: number } | { kind: "a", value: undefined } | { kind: "b", value: undefined };

function test(obj: { kind: "a" | "b", value: number | undefined }) {
	let x1: Foo = obj;  // Ok
	let x2: Foo = { kind: obj.kind, value: obj.value };  // Error, but shouldn't be
}

Above, our type discrimination logic reduces type Foo to just { kind: "a" | "b", value: number } because it is the only constituent of the union to which the type of obj.kind is assignable. This unidirectional assignability check is a little too selective. We should really be checking whether there is any overlap between the discriminant type in the object literal and the discriminant type in the target--in other words, the "a" | "b" type from the object literal shouldn't eliminate the "a" and "b" choices in the target. It's a pretty simple fix. I'll put up a PR.

[RyanCavanaugh]

Why isn't TypeB written this way?

type TypeB = {
	kind: MyEnum.B;
} & ({ id?: number | undefined; });

[WGroenestein]

Why isn't TypeB written this way?

type TypeB = {
	kind: MyEnum.B;
} & ({ id?: number | undefined; });

As mentioned above, in the real world example were we encounter this, the "id: number" type branch can have additional fields. While when not providing an id (or undefined), you should not provide these. E.g.

type TypeB = {
	kind: MyEnum.B;
}
& (
	{
		id?: undefined;
	}
	|
	{
		id: number;
		commentId? number;
	}
);

[fatcerberus]

You can’t actually prevent a commentId from appearing in the id?: undefined case without exact types - see #12936

[RyanCavanaugh]

Bisects to #53709